When you push a WitherSkull entity with slime blocks upwards, it will blow up. This also apply to Fireball and such.

This doesn't happen if it's other solid blocks, the entity will just be pushed
And when it's pushed by the piston only, the entity won't move, staying at the original location

Steps to reproduce:
1. Summon a non-moving WitherSkull entity
/summon WitherSkull ~ ~ ~

2. Place Slime block below it, and place a piston facing upwards
3. Power the piston
4. WitherSkull should explode