This can be due to a number of things, one of the most common being stress; our bodies do suffer when we're stressed or frustrated.
If not this, sometimes it may be due to needing a cut or a little tic. My hair used to fall out alarmingly when I washed it or brushed it. I then got it cut for the first time in maybe a year and since then, I have noticed a big difference. I've even had comments that my hair looks much healthier and it feels it too.
Maybe try changing your products or getting a leave-in serum to give it a bit of nourishment if all else fails??