It's called use it or lose it. If you don't spend 100% of the budget then it'll get permanently reduced in the next round and you'll have to fight extremely hard to get that budget back later if it turns out you need it. So you spend the excess on whatever will continue to justify line go up.

This is just 100% accepted as a universal law of bureaucracy. As long as individual managers own parts of a budget, it applies to large corporations, non profits, and perhaps most of all in government.