There are a few reasons. The top of your diaper could have gotten folded out, exposing the padding. If that comes in contact with your shirts or pants then it will wick out the top. You could have it taped on too loose at the top too. Or maybe your diaper isn't fitted large enough to extend up to your waist line (near the belly button). If there isn't enough padding, and you point up, then you could literally flow right over the top. Lastly, you might also be using too thin of a diaper. If there isn't enough padding there to absorb a flood quickly, then that pool of urine will have to go somewhere. With sitting down, there is added pressure around your legs and the lower padding is being compressed so it has nowhere else to go and overflows over the top.
Generally speaking, so long as you are using a diaper intended for large floods, and it is properly sized and secured on, then you should not be getting this problem. I'm well endowed, point up, and flood way too often. If I don't wear a large confidry or northshore then I'll get leaking over the top too. With them, thos problem never happens to me.