as somebody who works in confined spaces often. the most likely cause is lack of oxygen. the degrading of crap in the hole uses up all the oxygen. you go in, you get stupid from lack of O2, before you realise you are in trouble you are to 'out of it' to save yourself.
this kills a lot of people every year.
thats one reason you often see the 'lazy' guy standing around 'doing nothing' on a construction crew. anytime you have someone working in a hole or other confined space you need someone to monitor them.