A lot of it is schooling and education; I was hearing that a lot from history teachers growing up, as if an union was an erosion for society.
In high school, I was part of the union at my job. Being a teenager, it didn't bother me as much.
When I worked at one employer that was doing shady shit to employees, they actually had everyone meet with someone from HR who showed us an union pamphlet, asked us if we spoke to anyone about an union, told us what an union does to our pay (in their words), and to report to her anyone asking about an union; also we weren't to discuss meeting with her about this.
Thankfully, I'm no longer there and the employer has since closed. But the anti union sentiment is to cover up misbehavior from management.