Why do my hotpockets always split down the sides?
Splitting is presumably because of steam building up inside and trying to escape, causing them to break at the weak points. I'm not sure I've ever had a hot pocket, but presumably since they're designed to be microwaved, it's possible to avoid.

Most likely the microwave is just higher power or more uneven than average. If the hot pockets are a bit too hot in addition to splitting, reduce the time. If you get them the right overall temperature and they still split, try cooking longer at a lower power (e.g. 4 minutes at 50%).  If there's no turntable, open the microwave and move it yourself halfway through. If no matter what you do, they split, try poking a hole at the top of one end to let steam escape.