Why the use of budgets as performance targets can lead to behavioural problems such as budgetary slack (budget padding)?

This happens in bottom-down approach of budgeting. Usually directors tend to avoid being restrained by low budgets or committed to high targets.

So they buffer the budget, including higher than their expected Opex or lower than expected sales volume or value.

It's a sort of self preservation, so they always over-achieve and get their bonuses.

A good budget planner will know to challenge those numbers based on analyzing past performances and direct discussion. Until a more logical understanding of the budget numbers can be reached.

Agreed with Mr. Shamel Rashad

agree with mr shaml rashd answer 

agreed with mr shamel