Guessing you might have been being rhetorical but I’ll give my take anyway… The stabbings were just the spark that ignighted it all.
They do not have a common goal but they are united in thinking immigration levels are too high, despite it being a net benefit to the country. From what I’ve seen they generally fall into 3 groups:
EDL/BNP/facists/racists who have been whipping up anti-immigration rhetoric forever. Emboldened by extreme language used by Farage, successive Conservative governments and no doubt rhetoric from the convicted felon across the pond. They blamed the stabbings (and every problem ever) on immigrants despite the fact it was carried out by an autistic kid born in this country.
Brexit/Reform voters who see themselves as “more centrist” because they aren’t as far-right as #1. They blame years of austerity, worstening living conditions and growing class divide on the idea “the country is full”. They thought Brexit was supposed to solve this and are now protesting because it hasn’t. They claim it wasn’t done properly so voted Reform but got a more centrist government instead. They want to make their dissatisfaction heard but as far as I know do not condone the violence. They might now know the stabbings were not linked to migration but the issue has escalated beyond that incident now.
Young people who are generally not very well informed on any of the issues and have been fed misinformation on their various social media channels encouraging them to join the protests. It’s the holidays, they are impressionable and angry. They think they’re part of some revolutionary movement or just drunk and up for some chaos so are out with their mates filming it all in their phones for the views. I imagine as they become more informed they will align with #1 or #2.