why collateral is required if you can explain me what would go wrong if the pension funds were not made to post collateral

They're trading something everyone has to post collateral (a/k/a margin) on. This usually involves leverage, e.g. if they're buying futures [1].

The collateral is there for the broker and clearinghouse to sell if the pension fails to hold up its end of the deal. When the value of the collateral falls, the broker demands more to ensure they're covered.

[1] https://www.cmegroup.com/education/courses/introduction-to-f...