Hi @noeffingidea
I've got a pretty good idea. The Wikipedia article on the travelling diaspora is very well written and goes in to some detail as to what might be at the root of negative stereotypes.
There is good and bad in every community. I'm white British Caucasian and I've far, far, far more unpleasant/abusive and upsetting experiences at the hands of other white British Caucasians, and that takes into account that my friends are from a pretty broad pool of people (including Irish travellers).
I come from a rural family of farmers, and my grandad, for decades, allowed two of his small paddocks to be used by travellers along with water access. Not once, not ever, did any of us have one bad experience with travellers during that period. I asked my mum (who has a better memory of this than me) and she can't remember anything of note happening. These visitations by the travellers occurred upwards of six or more times a year, several nights at a stretch. Before I get "told" by someone else on here that they were circus workers or a "different" kind of traveller. No, they weren't. They were Irish Travellers from the traveller diaspora. So, no, not by a long shot are "99% of travellers" to be condemned as awful.
Gay people used to get a horrible, horrible time of it for decades. Many bad stereotypes were attached to them too. The same with Jewish people. The same with Transgender people. And so on. It didn't make any of those stereotypes either fair or true. Some people, like the poster who left the "99 percent of travellers" comment just want someone to hate. It makes them feel better about their own inadequate lives.
But anyhoo... I've seen enough hatred on this thread to last me several lifetimes.