beebellkel, it depends. How well do you remove shed hair? Like how often do you comb? If you aren't meticulous in removing up to a hundred hairs DAILY then it could be that when you are blowdrying is the one time you are meticulous. The straighter strands also mean loose hair can slide out easily.
When my hair is loose I comb it AT LEAST twice a week so I don't see any weird shedding on a day when I do anything different. But if you don't run a comb through your hair on a daily basis as you do when you blowdry, then that explains why you see all that hair. It's hair that had been caught and finally gets removed.
If you normally comb your hair daily and get 50-100 hairs coming out each day but more when you blowdry, then something else is going on. (BTW if the comb you're using isn't seamless, it could also be ripping at your strands. Even if not pulling them out, it could be tearing at them. You're better of doing the chase method with a seamless comb.)