Everyone leaving the train car at the same time?

I'm sure this is just some unwritten social rule that I haven't learned yet. I have only been living here for 6 months, so I'm still learning all the customs. But this feels way too weird, It's why I'm posting it here. 

When I'm coming home from work at around 8 pm, everyone in the train car will get up and walk to a different one. Nobody say's anything. They dont even look at eachother. After everyone moves it's like nothing happened. It doesnt happen all the time. It's only happened twice since i started taking that train. But it's so creepy. 

Like I said, I'm sure its just some unwritten rule. Japan has so many of them and I've been embarassed on many occasions when I dont know something. That's why I'm inclined to follow the crowd if everyone moves. 

If anyone had any idea, I know it's a longshot posting it here. But please let me know if you know whats going on. Cuz this is actually so creepy to me.