It's spring, the weather turns warn, and for the first time in many months all the cute people wandering around the street/mall/food court/city park/farmers market/etc. are wearing light clothing revealing faces, arms, legs, cleavage... The more of them distract you, the more often you turn to look, the more likely you are to get bisexual whiplash.
Straight people can get it too, of course, but being bi doubles your vulnerability.
The cute people are out today! I went to get some food and got bisexual whiplash just trying to cross the street.