You are certainly free to guess that the committee did something inappropriate. I have seen zero evidence of that though, so your guess seems quite unsubstantiated. But again, you are free to guess really anything.
The first part of your post makes it seem like Trump only threatened to prosecute the J6 committee. That is ridiculous.
- He threatened a ton of people during his campaign.
- He threatened to prosecute people that were part of his old administration.
- He threatened to prosecute elected officials.
- He threatened to prosecuteprivate citizens.
- He threatened to prosecute, military and government law enforcement.
- He threatened to prosecute judges.
- He threatened to prosecute lawyers.
He quite literally sued someone for fraud, simy because thry published polling numbers that showed he was trailing in Iowa shortly before the election.
He ran a campaign based on personal vengeance and retribution.
So I am guessing people were concerned he wofoluld low through on his many threats against them.