There may have been inter-breeding involved with wild populations, but most, if not all, feral cats have their origins in domesticated, or captive (by humans), cat populations.

Also, have you ever noticed that feral cats tend to congregate near humans? That's because they find it easier to find food from dumpsters, handouts, garbage, etc. This is very similar to other species that rely on humans for their success (rats, corn, cockroaches, crows). This artificially increases their numbers. I say "artificially" because their food source comes indirectly or directly from humans.

So simply from the definition of the word "feral", and not even including the inter-species dynamics between humans and feral cats, humans have everything to do with feral cat populations.