When using an SSH key with a passphrase, if SourceTree is open for a long time (e.g. all day), the SSH key is initially held open, but at some point appears to timeout.  After that, if you try to push, any hooks run, but the push just hangs until it eventually times out, but doesn't prompt for the SSH passphrase (or clearly state that's the problem).

To workaround, if the push appears to be taking too long, cancel the push, then open and close options, which re-prompts for the SSH passphrase, then re-push.