Deep tissue massage DOES cause severe migraines. I used to be a massage practitioner and could never get massage because it gave me horrible ones, no matter how much water I drank. I found that about 5% of the clients in my clinic reported them.
I don't think it's because of the old "releasing toxins" theory, as it isn't scientifically sound. I haven't yet read a good explanation from the studies that demonstrate why it happens.
I had to quit getting massage completely and just get PT. I tried over a dozen types and nothing was any different, no matter what anyone claimed. I tried hot packs, cold packs, gentle stretches, everything imaginable after. The severity of my symptoms did seem to diminish the more massage I had, but but headaches always returned. I never get massage any more, and I don't give it any more, either!