We get tagged for any number of reasons but the most common ones would probably be.

1. They smell feeders and mistake a nice warm heat signature on a hand as a prey item

2. They don't feel good, sick, wrong temps or frightened or they are startled by a hand.

3. They are young and more prone to being fearful and to strike out.

Another reason would be the individual behavior of each snake, some snakes are just bitters.