Millwall lost in the last minute to Birmingham City (who we loathed for good reason) and this caused a very tense atmosphere. When people were leaving the ground and were heading towards The Blue, the police put a line of police in full riot gear across the road causing a big panicky crush to develop. This crowd were still on the whole good natured and had many kids on dads shoulders, blokes with their wives and girlfriends etc. Then the police charged the crowd (who had no where to go) and batoned loads of people including kids on dads shoulders etc causing loads of nasty injuries and creating a palpable sense of rage and fury.
This mindless attack by the police on a largely peaceful crowd was the REASON that many people then took their kids and girlfriends home, came back, looted the adjacent scrap yard and pelted the police with the contents repeatedly.