It happens..... it happens to everyone.
Like BI said, it could be a nervous system thing ( but not CNS, that term is misused) - and you just need to handle bigger weight more often.
I get it more when I'm depleted or when I'm not focussing on pushing my abs out in to my belt. Weak abs can have the same effect... usually a small muscle weakness in the chain used for the lift is the culprit.
Sometimes it's just mental - because a weight feels heavy you're just squeezing super tight so you shake.
EDIT - I just read JENNER'S post and can definitely agree with that as well. I have really tight hamstrings and me neglecting them can bring on a shake too. Good call JENNER