Steam can do pretty well filling a tail circuit, probably better on average. But a torrent of a large file with a ton of peers when your client has the port forward back into the client absolutely puts more pressure on a tail circuit. More flows makes the shaping work harder.
Sometimes we see an outlier in our reporting and it's not obvious if a customer has a torrent or a DDoS directed at them for the first few minutes.
Your bridge isn't bridging properly. If Router B is sending a destination unreachable then the packets are being handled on it further up the stack at layer 3 by some sort of routing component rather than by a layer 2 bridging one.