It's happened to me a few times when I've been in a sitting position and my nappy has been quite tight and I've had a fairly large accident. It almost seems like the liquid fills the nappy and will leak from the easiest place, which in my case is the top!
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.
It's happened a few times, though not as often as it used to. When my parents handled my nappy changes when I was a kid (until I was about 13 or 14) it happened much more so I think it's something to do with them not taping them tightly enough, they also had a habit of getting nappies a little too big for me.
It does happen, but the chances are reduced if you persevere to find the type of diaper that suits your needs best. This is a matter of trial and error but some highly regarded brands leak at the waist band on me andsome cheaper ones don't. I use a belted diaper as it is easier to fasten and to get the waist snug. The velcro fasteners allow for easy adjustment and don't stick to your body like tapes do if they get in the wrong place.