Incumbent women face more challengers than incumbent men
As Geoffrey just mentioned, Summer Lee is leading her primary challenger, Bhavini Patel, in the 12th District. Political science research actually finds that incumbent women in Congress are more likely to face challengers than incumbent men. This is due to several factors, such as a history of women's underrepresentation which gives the impression that female incumbents are more vulnerable; this leads to higher quality challengers, too.
—Meredith Conroy, 538 contributor