Bleeding Heartland did a story about this very question recently.
How could this happen?
...WHY CAMPAIGNS ALWAYS COLLECT EXTRA SIGNATURES
Ask any experienced campaign staffer: you always want to submit far more signatures on nominating petitions than the law requires. Inevitably, some signatures will be struck because they are duplicates, or because the voter made a mistake like signing a petition for the wrong county.
...Finkenauers staff didnt respond to several messages over the past week seeking to clarify why they left themselves so vulnerable to an objection.
Adding to the mystery, the Democratic front-runner turned in her petitions on March 10, eight days before the deadline for submitting nominating papers to the Secretary of States office. Why wouldnt the campaign spend that last week padding their totals in counties where they were barely over the line?
One theory Ive heard: the candidate and her staff misunderstood the legal requirements. The election law Republicans enacted in 2021 more than doubled the mandatory signatures for U.S. Senate candidates (from 1,500 total signatures from residents of at least ten counties to 3,500 signatures including at least 100 from nineteen counties).
Although Iowa Code and the Secretary of States primary candidate guide spelled out the new requirements, its possible Finkenauers staff thought they needed to meet the same threshold as other statewide candidates: 2,500 signatures including 77 from at least eighteen counties. If you were shooting for 77, then having a bunch of counties just over 100 signatures would look like a safe margin.
https://www.bleedingheartland.com/2022/04/03/how-could-this-happen/
She's got almost 2 million dollars and seven paid staffers. Somebody seriously screwed up.