There are contradicting reports that West Ham fans did it as well.
Some journalists who attended said they heard it and others say they didn't.

Whilst it doesn't help with The reputation of these two clubs I'm going to give them the benefit of the doubt and say their motives are more down to objecting to conforming and being told what to think.
There are also question marks over some of the less obvious underlying motives of the BLM.

As someone has mentioned the kick racism out campaign has more gravitas and credibility for getting the point across