Why does the sulfone ring in tazobactam open when the lactam is hydrolyzed?
Here's a draft:
First, read this paper. Considering we're under alkaline conditions (blood pH between 7.35-7.45) we'll take their alkaline mechanism proposal.
Mechanism

This is obviously incomplete, with good reason. Without knowing what the active site of the enzyme looks like (except for Ser70), it's difficult to devise of a mechanism. I could imagine another alkoxy attacking the sp3 carbon between S and N, and shifting two electrons to the sulfur thus generating your SO2- in one step, but that would require a proper active site.
The geometry of the molecule inside the active site of the enzyme is obviously optimal for this reaction to proceed. I'd like to know more. For some reason my school doesn't give me access to this paper.