Why do some champion monsters split?
Solution 1:
Some champions have an attribute called splitting. Upon death, they split into 2 weaker versions of themselves. If you hover your mouse over the champion it'll give information about them (knockback, leaping attack, etc.) and some have extra information, usually at the bottom of their name in purple saying "splitting" or "haunted"
Not all champions have the splitting ability and it's not a bug. It's random which champions have this attribute.