We say spring, summer, autumn, winter as both school year and fiscal year start in April.
Most of Japanese people have an image of spring being timing of start. Hundreds of years ago, people called January to March as spring because it was the begining of the year.
I think it is a bit awkward to use 「一触即発」for describing one's personality which is short-temper, getting upset easily, etc. since the expression is normally used to describe that something dangerous/unwanted will be triggered on the spot when an action taken.
I guess the word more suitable is「瞬間湯沸かし器」: instaneous water boiler or 「すぐカチンとくる」: easily losing ...