Why did two of four bolts get damaged?

[HangmanNY]

I’ve had the front end pulled apart to clean up rust. I removed this cross frame brace/support (near lower control arms). Just to remove road gunk and rust. The sprayed with rust reformer. Today I’m paying with black enamel. May use fluid film after at some point.

Two of the four bolts were rusted, threads damaged . The other two were fine. Is there a reason why these two bolts were damaged? I just ordered two more of them from a Mopar supplier.

Thanks for any helpful advise!