This only appears to be an issue when the vehicles have close top speeds (i.e. MPS Bus & Mail truck, which have 30 & 35mph respectively (48/56kph))
I think, though am not certain, that the "special moment" happens when the overtaking vehicle crosses into the next tile, sees that it is clear, and stops overtaking. Of course at more or less the same moment the overtaken vehicle crosses into the same tile and we get the intersection of vehicles seen here. Presumably this also happens at faster overtaking speeds, but is less obvious. No idea how the vehicle is allowed to "overspeed" beyond its top speed though. Probably something to do with resolving the intersection.