from teh article:
"He suggested men might respond aggressively because they see the
other unicycling man as a threat, attracting female attention away
from themselves.

"This would be particularly challenging for young males entering the
breeding market and thus it does not surprise me that their responses
were the more threatening."

More complete discussion:

http://www.bmj.com/cgi/content/full/335/7633/1320

I always assumed it was just my good looks - now it seems they were
only after my unicycle.