rottie bit 5 year old girl

okay folks,
I have a 14 month old Rottie, Enzo. I was walking him through the neighborhood yesterday on leash when a little girl on a razor scooter was riding by. as she was passing Enzo he lept out and bit her on the arm. She only had minor abrasions.
This surprised me to no avail, he had never done anything like this before, that I knew of. It wasn't until I told my boyfriend about this incident that he told me that he lept at a little boy on a bicycle a few weeks ago.
Guys, what can i do to deter this behavior, Enzo is an otherwise easygoing, pup. He does have the tendency to jump on me when he gets excited, but never bites me.
this is my first rottie, and i don't know how to correct this aggressive behavior.

p.s. he has not been neutered yet

Year's ago we had a OES that hated anyone on a bike, don't understand what caused it, but Shona might be able to help.xxxxxxxx

Did the scooter come from front or rear,, many rotties have a high pray drive, I had to do a fair bit of work with holly, when it came to things like this, I never had an incident but new it was a possibility with her and worked it through before it ever happend, I would take him to area's with kids on bikes and so on but let him see them coming, try to avoid them scooting up the side of him and taking him by surprise,,, its easy to solve, if you prefer to pm me thats fine,, also if you have any kids in the family it could be good to work it through with them first, I think from what you say though,, hes not used to seeing this sort of thing, or he got a bit of a startle, x

the girl came from the front. she was a ways away, and Enzo saw her approaching. she wasn't very close to him.
my boyfriend has a six year old son who Enzo gets alog with quite well

Think Shona has hit the nail on the head. Different breed I know,,,,and herding instinct, but we had this with Skye, except she was grabbing at the bike or scooter....even managed to bag herself a Police Officer We've worked through it by doing as Shona said, now wehn they come along we say leave before they get close, herding instinct still there, but she lets them go by now.

I had the same sort of problem with my rescue rottie, he would lunge and chase at joggers and cyclists. The way we dealt with very similar to shona's approach but as my rottie is so food motivated I added a treat into the equation - we would see a jogger/cyclist and I would immeidately put Murphy into a sit position and keep feeding him high value treats until the jogger/cyclist went past. we would continue to do this for about a week then I woudl slowly decrease the amount of treats given so eventually he he only got a treat when the jogger/cyclist past us.

The day I knew the penny had dropped was when Murphy was off lead and I didnt see the cyclist but Murphy obviously did and parked his bum on the floor straight away and looked at me for his treat I was so proud of him.

Even now though, I know there is always that potential that Murphy may chase so I am always on the look out for joggers and cyclists and when we see one I always ensure he is in his sit position and close to me and we have had no incidents

so many rotties live for the thrill of the chase,, some however just want to dive on things,,,this is more of a problem than chasers imo with rotties, the ones that like the chase get the kick form that,,, the ones that grip when its near to avoid chaseing are the ones who get the thrill from the grip,, the latter dog needs far more assertive training and handling, they can go on to be a big problem,

there are also the ones who got a fright because something came from the back of them and its just the reaction to getting a fright,, these dogs can become a problem if they get such a fright that it goes on to become a behavioural issue, but most dont,

MOST rottie breeders would like to sweep the HIGH pray drive under the carpet with the breed,,,ITS A REAL ISSUE, you must be aware of it and have the correct knowlage to deal with it, Quickly x

Right so he would have fixed his eyes on it,,, THATS you que to step in and {tell him off} move is attention to something else,, I would put him in a sit,,, but let him look at the scooter,, to try to drag his eye off it would only be avoiding the situation,, I guess I would let him stay facing the scooter but gain his attention, like stephanie said,, get him looking at you for food a toy or whatever,, dont allow him to jump around,, have him on lead,, if he messes about any you had to hold him back, dont feed him because the scooter got by,,,only feed him if he does as he is told, so many people feed dogs just because the dog didnt connect, even though the dog was trying like a bear to do so and the handler was hanging on for grim death, to feed in this situation is really training the dog to be a yob,,, im sure you will sort this out quick though x

thank you all so much for your input. very invaluable, and i will definitely institute more firm/observant practices beginning asap.
i love Enzo to death, and it just pains my heart that he bit that little girl, but I also know that most of the blame lie on me; I will work to the nth degre to become a better owner.
thanks so much everyone.