Maybe he could feel a change in the weather? Dogs are very receptive to changes of weather: thunder, heat, cold etc. Make note of what the weather was like when he was restless and see if the same weather conditions are coming up and monitor your dog to see if he starts again.

Thanks mirandashell and brenda1, he's fine already (since last Saturday). Anyway, I don't think it was the weather...

Good to hear he's back to old self