Okay, don't usually post on this type of event, but it appears we have two aircraft on crossing track ,one descending to 5,000 one climbing to 4,000. The loss of separAtion appears to have occurred after the crossing point,so at the time that vertical separation was lost the aircraft were diverging so horizontal separation was building. The cause of the alt deviation could be one of many things as already stated -
A) a failure to capture the correct altitude
B) incorrect altimeter setting meaning that the crew believed that they were not yet at their assigned altitude
C) crew reacting to a TCAS resolution the resulted in the aircraft passing through cleared level but ensuring that the aircraft never were going to collide
The thing to remember in C) is that TCAS is aimed at preventing a collision not providing separation. This scenario is not uncommon when you have high performance aircraft climbing/descending and crossing.