Why bleeding after taking the morning after pill is a common side effect?

Hormone: The morning after pill is commonly associated with bleeding as a side effect because it is made out of a substance that is very similar to the body's own natural hormone that's responsible for menstrual bleeding.

Hormone pills: Emergency contraception pills are a one-time bigger dose of hormones... meaning the effects are temporary but are disruptive to the normal cycling of a woman's hormones. That means there may be some vaginal bleeding irregularly for 1 or even 2 cycles, because vaginal bleeding in this case is from changes in the lining of the uterus, related to hormonal level changes in the body.