I think for that split second, a lot of them were taken up in the 'heat of the moment' and if you read reports afterwards when they were prosecuted (ie caught red handed) many were quite remorseful because this was so wholly out of character for them.

Major part of it for some (and many were quoted saying as such) was they did not believe they would get caught and if they were they would not be punished  very severely so it was worth the risk. And the reason they were out in such force was due to social media - many people were prosecuted for using twitter etc to incite riots and vandalism.

'fraid to say it was probably nothing more than 'herd instinct'