When I was a vegan for around two years (my stores of vitamins didn't last very long), I was always a very angry, emotional person and I got angry over the smallest things. Looking back on it, I now realize that I was incredibly deficient in B vitamins. I even had clumps of hair falling out, cheilitis, and patches of melanin-lacking skin.
Eventually it all progressed to the point of me having to be hospitalized. By that point I was having hallucinations, delusions, and shooting pains stretching from my hips to feet and shoulder to arms (a hallmark of B-vitamin deficiency). I was also severely anemic.
When I talk to another vegan, I find them getting very angry about many things, whether or not they're important or not. I always just write it off as B-vitamin deficiency, or some other vitamin they may be lacking.