Not sure which year had the most but I know a few reasons why it feels like it in 2017:
1. Higher number of cards. Higher number of injuries. Simple math. Weekly cards mean weekly injuries.
2. Weight cutting. Fighters are cutting more than ever. This results in more health issues with cut. We have seen many pullouts like Khabib, etc.
3. Money/legacy on the line. Fighters these days are more hesitant to take a risk. Fighters of the past fought more frequently and were less scared to lose therefore they likely were often injured going into their next bout. Today fighters are more likely to pull out over minor injuries. They only want to come in 100%. Every loss these days could possibly mean them getting cut (HUGE UFC roster) or not getting a title shot for the rest of their careers.
4. "New innovative training methods!"
5. USADA
6. Tiramisu