I’ve seen this too, but the cause is not always obvious. I find typically that one vehicle wants to make a manoeuvre which conflicts, and it halts, and everything around it gets stuck. Working out which one is a challenge. The ‘asleep’ cars, I observe, are often waiting to do a u-turn but can’t because of the other traffic. Sometimes they do seem just to go to sleep, though!
I tried the cobra tram last night and it just got stuck in the (large) tram station, tangled up on entry/exit. The vehicle interactions are too strict and need to be relaxed.