Video: https://www.youtube.com/watch?v=18AxBV2uDB0

It's a wrestling match at a smaller venue, so instead of having permanently installed benches or chairs, they have a boatload of folding chairs for all the people. The average person has the good sense to not throw their chair into the ring, through a mixture of morals and fear of punishment. But if they feel their action is justified (one of the wrestlers calls for someone to hand him a chair as a weapon, which happened in one incident) some will throw. As the first few people throw in response to the call, others, less fearful of individual punishment, join in. Sometimes this results in an avalanche of chairs being thrown into the ring, burying one or more wrestlers or ring personnel. Fortunately, as the chairs are relatively light, the wrestlers can spare themselves from major harm by hitting the mat (as seen in the gif) and letting the chairs pile up, sparing them from being directly hit by the majority of the chairs.

Reminds me of this: