So the bomber gets hit, and the control cables are severed, it could go into a g-inducing barrel roll type spin. Or it could go into a flat spin while losing altitude. In that case, the crew would be pinned to the roof. In the numerous accounts I have read, crew members recounted being stuck inside the bomber unable to move to the exit. The only thing that saved them in these cases is the break-up of the bomber, at which point they were either thrown out or could somehow escape.
But the main point is that G-forces played a huge part in bomber crews not being able to escape. Whether negative or positive G's.