We'd really need traceroutes (ideally bidirectional) from before, during, and after the break to diagnose your issues :P

But, even if your traffic was going east, with the broken cable to the east, there might be a lot more traffic going east (or coming from the east), and that could cause a lot of breakage.

For better or worse (mostly for worse), BGP doesn't propagate capacity of links, so it doesn't matter if there are alternate routes, if the overloaded route has the most desirable advertisement, it gets the traffic even if most of the packets are dropped into the sea.