Anything can trigger it, not enough sleep, feeling the need for control, not wanting to be wrong again after a string of losses, trading too big, being hungry, a hot streak etc.
The key to avoiding this is following your rules, and to be SELF-Aware of your mental and emotional state at all times. If you feel yourself wanting to deviate from your predetermined plan after you have placed your initial trade or you start looking for reasons as to why your trade has moved in a certain direction you need to be careful as when this happens your rational judgement is going.. going... gone...!
It can even cloud the judgement of seasoned traders and if you feel it coming on you may as well close up shop for the day and save your account anymore damage, because you are no longer trading with your best interest at heart
Watch this video and dont make the same costly mistakes.