Trump's 2016 campaign targeted Crooks parents as potential supporters due to their interest in guns, which they probably got a lot of pro-gun messages from the campaign in both 2016 and 2020 in the hopes of getting them to vote for Trump:

Trump campaign secret data on gunman's family

The gun-related profiles which appear in the 2016 Trump campaign database range between 0--meaning no likelihood--and 1--a certainty.

Matthew Crooks scored highly across a range of gun-related models including: 0.99 for the likelihood of returning a warranty card for a firearms purchase; 0.95 for the likelihood of pursuing hunting sports; and 0.94 for the likelihood of shopping at the hunting retailer.

Overall, averaged across all three of these measures, out of more than 19,000 people in Bethel Park, Matthew Crooks was among the top 20 highest scoring individuals.

"It makes me wonder why he would carry out an assassination attempt on the conservative candidate."

We don't know Thomas Crooks motivation. If he was a Trump supporter who turned on Trump that would not be an unheard of development. In an article I posted yesterday history professor Timothy Snyder esplains:

We might be tempted to think that violence against one side must come from the other side. But the bloody genie, once unleashed, often stays close to home. Those who have made violence normal are especially vulnerable, because they will always have colleagues or followers who think they have not gone far enough.

So it seems this info (from the above link):

"The programme can now reveal the information was compiled as part of a secretive project aiming to identify millions of gun owners in America who could be targeted with pro-gun rights messages in the lead-up to the 2016 election campaign."

coupled with this reporting:

Local Pittsburgh News WPXI: Crooks' motive is still unclear. Records show he is a registered Republican and neighbors today told us that they've actually seen Trump signs outside of the home over the last few years

Hmm, you can see that someone(s) in the family were enthusiastic gun owners, and you can see that Crooks' father suspected his son was the shooter and notified police:

More than a dozen guns were found in a search of the Crooks family home, four senior officials told NBC News.

After Crooks opened fire, his father called police to say he was worried that his son and his AR rifle were missing, three senior officials said. Police went to the home after the call.