I think it's mostly loss of felt that causes light-leaks, though if the cap starts to spread a bit it could leak around the corner where the film entrance is. Keep your cartridges in canisters if possible to further reduce risk, especially when there is no leader poking out.

In order to minimise felt loss, I mark half my cartridges as Source (load with fresh film) and the other half as Sink (exposed film goes into these), which means film is always flowing the same direction over the felt and not bending it back & forth. Maybe that helps, maybe it doesn't - the main thing is don't buy really beat-up old canisters.