From the handbook: A vocal or rowdy crowd reacting to a worker can happen with certain products if the worker's key skills are too weak.
For example, in a deathmatch based product the fans would react if the worker's brawling and hardcore skills are low, with the lower they are, the worse the reaction.
I usually search for workers who have 45 minimum in brawling or hardcore so they don't get booed out of the building due to the product
Hope this helps.