He's been wanting to get rid of Sessions ever since Sessions refused to recuse himself but Trump was reluctant to do so, because it would be viewed as too obvious a move to try to punish a member of his own cabinet that he deemed disloyal.
Now that Trump is smarting from a stinging loss in The House of Representatives (no matter what he claims, he's upset by it), he's unhinged and no longer cares about public perception.
That's why Democrats are being foolish if they don't proceed with an investigation. Trump and the GOP don't care on whit about civility or fairness in rules of engagement, they certainly don't care about being nice or conciliatory.
Meanwhile Democrats just got the House and are already wringing their hands about whether they will proceed with a basic, but entirely necessary investigation.
I hope Rep. Maxine Waters requests those tax returns on Day 1 of her chairmanship-- Trump/Republicans have certainly threatened to investigate Democrats (including lawyers and witnesses to the Kavanaugh hearing) for everything.