It's really not a fact. It's an opinion. No one has suggested the Proud Boys woke up one day and decided on a whim. They have been stoked up by Trump and bigots who have a reason for this. They have been getting stoked up for years and then been lied to that an election has been stolen. To try and lay part of the blame on the BLM protests just to support your own beliefs they were wrong is pretty simplistic.
Tubey has a point that these riots didn't happen in a vacuum and are reactive to something.
However, it is ironically a vacuous point as, of course, literally nothing happens in a vacuum and everything is reactive (that is how linear time works).
Equating the BLM "riots" on the streets of the US, motivated by an attempt to throw off the shackles of oppression and racial inequality, with the literal fascist assault on the heart of US democracy motivated by having lost a democratic election, is a spurious line of reasoning.
