As I understand it (as a regular on the ZeroMQ and Crossroads mailing lists), the reasons are as follows:

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").

If you want more background to what led up to our fork of ZeroMQ, read the flamewar on zeromq-dev from February carefully hidden in a "C++ assertion" thread at http://lists.zeromq.org/pipermail/zeromq-dev/2012-February/0..., or from http://lists.zeromq.org/pipermail/zeromq-dev/2012-February/0... which relates directly to the trademark question.

I noticed there's a compatibility library with 0mq. What are the technical changes that require a compatibility library?

This seems to imply there are API differences, rather than just the same code with different development policies going forward.

The compatibility configure flag seems to be used to also install the crossroads library as libzmq.so.

"To grow such an ecosystem the project must be fully vendor neutral, and implement a liberal (e.g. Linux-style) trademark policy allowing use of the trademark for third party distributions of the software, as well as for plug-ins and extensions."

Ah, a Centos/Red Hat thing.

I'd say “personal differences” (regardless of the deal about trademarks and code review), but it still surprises me; I thought the zmq developers were a tight-knit bunch. Maybe the drama manifests itself in more extreme ways because they know each other so well, but it does seem dysfunctional.