An added distraction. Someone going more slowly, seemingly impeding the progress of the others on the road. Someone who shouldn't be there on the road, getting in people's way. Someone smaller and ostensibly less capable of defending himself/herself. Probably any number of "motivators" for folks who think that way, depending on the person.

The same essential frustrations occur when others are "impeded" by slower drivers. Horn honking, thumbing, threats, cutting off folks, hard-braking, even in some cases (which I myself have been a victim of) attempts at shoving the other off the road despite being in a vehicle just as large as theirs.

Puerile, dangerous acts by mentally juvenile "adults" who are permanently stuck in the third grade. Only, when it's lashing out upon cyclists or pedestrians it's people's lives on the line.