There are no doubt infinite possibilities for mishap with these powerful, heavy, awkward, machines designed to have multiple things all happening at once and with numerous detail that must be attended to. Their surfaces are also very hard. Much harder than buildings, plants, bodies, etc.
Combine that with human operators that have limited abilities to multitask (some say none), minds that continually wander and distract themselves, narrow ranges of perception, senses that can get skewed when overlapping, varying levels of experience, often questionable judgement, and you just never know what will happen next. Furthermore, as the rate of motion increases and potential for intended outcomes decreases. The good news is that as the operators age they tend to slow down. Downside is that their functional abilities tend to deteriorate.
Then there is the unknown outcome of having had an argument with wife (or husband) or watching the news just before jumping on the tractor. Forgot to mention above the effects of the volatile emotional natures inherent in our natures which distort and distract mental and sensory functioning.
I am very grateful that it all works out as well as it does.