organic chemistry - Decarboxylation mechanism and stability of α,β-unsaturated and α-hydroxy acids

I was wondering why α,β-unsaturated and α-hydroxy acids undergo decarboxylation. Also, I'm aware of neither the products nor mechanism. All I know is that β-keto acids and α-nitro compounds have resonance stabilized anions and have stable transition states.

But α,β-unsaturated have none of those and it isn't that electronegative as its sp². Someone please help me out; I can't find the answer anywhere else.