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Right today I met Andrey Ozerov (one of the best world specilist for Sepsidae) and put him this question. His answer was - mating behaivor. Reasonable.
Pro this explanation is that all Sepsidae, Opppomyzidae ect have more or less spoted wings. To move spotted wings is the best way to show your size both to female or another male. But it is for my oppinion still unclear what is first and what is secondary - wing moving or spotted wing.
Contre it is that, it seems to me, that I've seen wing-moved females too.