```Why was Crossroads I/O forked from ZeroMQ?
The contribution and trademark policies of the ZeroMQ project were, as of March 2012, incompatible with our long-term goals.
Who is behind the fork?
The main work on the fork was done by two of the original developers of ZeroMQ, Martin Sustrik and Martin Lucina, with help and advice from other members of the ZeroMQ community.'''
1) Trademark issues - the original developers of ZMQ have been prohibited from using the ZeroMQ trademark and, thus, from making official releases.
2) The new ZeroMQ contribution policy[1], in which pull requests are merged without review and then later reverted/enhanced if necessary, is seen as incompatible with the goals of some of the lead developers. These developers prefer a more rigorous review policy.
There are more reasons, but from what I can tell, those are the main ones that haven't fully been covered here.
[1]: http://www.zeromq.org/docs:contributing (see "Philosophy").
This seems to imply there are API differences, rather than just the same code with different development policies going forward.