As a complete layman, I've always assumed hybrids are a result of two recently diverged lineages. If they're both still near the "fork in the river" so to speak, they have enough similarities to overcome the differences that are building up.
As far as I know, most hybrids are pretty much forced, either in a lab by humans, or by extreme environmental factors, so that in the wild, in most cases, these animals are reproductively isolated from one another, and as such, we can only assume more changes will develop unless we artificially hold them in their current forms. If and when these changes overpower the similarities, hybridization will no longer be possible.
But again, I'm so much a layperson on this that I'm downright recumbent.