Tippmann 98

A marker uncocking itself can be caused by a variety of problems.  A lot of the time, it is caused by the co2 tank being low, or being at a low temperature.  It also depends on how fast you are firing, and the position you are firing at.  If you fire extremely fast or with the barrel pointing downward, liquid co2 can be sucked into the marker through the gas line, leading to a sudden drop in pressure.  It's normally not a huge problem, seeing as most of the time on a field you aren't firing at a downward angle. 

When it uncocks, does it just uncock or does it make a sputtering, fading out farting noise?  If it is sputtering, it could be a sear problem.  Disassemble the marker and check the sear for wear.  Tippmann may send you one for free if your marker is still in warenty.  If your marker is not, it still shouldn't cost very much at all.  If your marker just uncocks, refer to my first paragraph.

Basically, make sure your co2 is full.
Make sure it's at room temperature.
Point it horizontally when you shoot.