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The habit of males mounting males reminds me of the genus Chasmatonotus of Chironomidae. This genus is winged but does not like to fly. It probably evolved on isolated mountaintops during an ice age. According to a paper I read,

"The adults scurry around on the leaves of broad-leafed plants, shrubs and small trees within the habitat sites. When two adults encounter each other they 'tumble' together for a second or two before moving on. As the adults do no swarm for the purpose of mating, this behavior is probably associated with identifying a possible mate."

Possibly the habit of mounting everything in sight is typical of Diptera that do not fly.