This happens when any plan step of a reasoning method (e.g. body()) takes an unreasonably long time to complete (about half a second). It usually means that something is calling out to Java code and not coming back, normally a bug with your code. Individual plan steps must execute quickly. In any case, the JACK® kernel leaves that thread alone since it is taking too long and starts a new one to handle other plans. If the original one ever comes back then all is ok. If not, then the agent mentioned is hung.
Often this is harmless but it might indicate a bug in your code. There are several ways you can include a lengthy computation within a plan step should the need arise. One quite flexible way is to use an Action cursor (aos.jack.util.cursor.Action). See the JACK Agent Manual for details.