I could see a conflict of interest and I'm disappointed she didn't recuse herself voluntarily or pick someone else to be involved in the case.
Appearance is everything in this high profile case and it needs to be by the books or else half the country will scream "unfair, rigged, collusion"
The district attorney could have feelings left over from the affair and could possibly want to influence the case to see her lover win.
Edit:
Okay I'm being down voted.
Preface: I hate Trump as much as all of you. I think he is a treasonous rapist lying twice impeached failed businessman dumb man child.
However, consider the [impossible] hypothetical scenario where an anonymous source leaks key information specifically to the DA proving Trump's innocence. She is obligated to disclose this information to the court.
However, if she has plans to marry the prosecution attorney and wants him to be famous for winning the case against trump, she might not disclose that information.
These are the kinds of mental gymnastics the Republican party uses to justify throwing the case out, and this is why she should have recused herself.