People Are Stupid: without some good guidance (rules, "adult supervision", tasers, etc.) people are going to do stupid things because they a) think it's cool; b) think it makes them look cool; c) they want to be on the leader board; or d) they want to be on YouTube. Without someone, meaning a responsible coach or trainer, to reign in the stupidity (and hopefully add some common sense and knowledgeable guidance to the situation) people are going to do things that get them hurt. If your coach or trainer is not advising you on how to scale each and every workout then you are training at the wrong gym; and if they aren't scaling the workouts for you what the the heck are you paying them for anyway?
Groups of People Are Stupider: get a group of people together in a dynamic and highly volatile environment and they are going to do stupid things; think Spring Break, mosh pits, riots and most CrossFit Gyms. The group environment can do amazing things for some people by getting them fired up and making them want to work harder and achieve more. For others it just ends up being a "d$%k measuring" contest and a "win-at-all-costs" competition. The coaches role in the group environment is crucial to ensure everyones safety (even if it means keeping some people safe from themselves) as well as to keep the "competitive spirit" in check. If your coach isn't stopping athletes from going too hard, too fast, and too soon then (once again) you need to look for somewhere else to train.
Check out this great article on the three reasons you get hurt at CrossFit and see if any of this is happening in your gym!