Question about CAN2 reactivation after connection lost

Hello.

I am developing on stm32f4discovery board and I use both, CAN1 and CAN2 in my project. I know that CAN2 can't be used without CAN1, but what I am struggling to figure out is that if for example, I lose physical connection in CAN1 for a while and a little bit later, the connection is reestablished, CAN1 will start working again, but CAN2 won't. I use a TJA1051 to convert can rx and tx messages to can high and low, and if I unconnect either can-h or can-l cable and connect it again, only then CAN2 will start working again. 

Why won't CAN2 start working again without reconnecting a cable. What my cause this issue ?