Bernie has always been a person who aimed to achieve as much as possible from the outside. He understands that the only meaningful electoral path forward for the left is through the democratic party, as third parties have repeatedly failed throughout american history, only ever handing elections to more right wing figures. That it's better to just treat the dem party as if it was a perminent coalition rather than a coherent party.

The only reason to behave nice is if you're getting something for it (and that for him is labor secretary), without that he has no reason to behave nice and he has multiple times now made that clear in various lefty media interviews.

Also the dude clearly had a personal vendetta with Trump that made him want him out.