I posted this in /r/explainlikeimfive before realizing that this would be a more appropriate place to post it. Because I want a legitimate, sourced answer...and because I don't think anybody needs to dumb down the explanation to a question like this.
Films and TV shows throughout the decades have crowds at theaters throwing tomatoes at poor performers, and angry mobs doing the same in public areas. Where did this start and why?
For tomatoes in particular, it started in the 19th century in Italy. Here's a previous answer you may like. Prior to that throwing things at performers (both nice and mean things!) happened though, and we still throw nice things in some instances, like throwing flowers and little stuffies etc. on the ice at the end of ice skating performances. Some places in opera still do some throwing (La Scala recently had some people in the top balcony throwing invective pamphlets down on the lower balconies to protest a new staging of an opera) but we don't eat food at the opera anymore so throwing food's pretty much out.