your memory is fading... these riots were quite often under trump. and they stopped abruptly when Xiden got frauded in. with no reduction of police brutality. now all of a sudden they are rioting again. that is my Why??
Antifa rioted under Trump because Trump was the leader of their enemy (a free America).
Just because they haven't heretofore made a lot of noise under Biden doesn't mean that have disbanded or have taken a vacation.
Their actions are always intended to further the cause of Marxism. That includes much more than rioting.
When rioting furthers their cause, they riot. They obviously have concluded that these riots will not damage their cause and will indeed further it.
Antifa were behind the BLM riots too.
They held the Presidency and Congress after the 2020 election. Currently, the Republicans hold the House. I think that we will see more of them for the next two years.