Because they're just lying right in the middle of Akihabara's main road (and in all kinds of other places), and Acquire took the trouble of photographing them and setting up a teaser site and twitter widget that allows you to tweet the pic along with the words "Akihabara x Trunks".
While I doubt this is quite like the anime Sora no Otoshimono's flying panties promotion, I figure this is somehow associated with whatever their latest project is going to be. Acquire's known for making quirky games like Way of the Samurai and the Badman series, so it could be pretty interesting.
If I were to take a guess as to the nature of the new game, though, I'd be thinking that a recent web poll they conducted regarding what sort of game fans wanted them to make could have something to do it. Then again, the respondents mostly voted for "a ninja game" (since Acquire made the Tenchu games before the series became terrible). What do ninjas have to do with mysterious underwear sightings? That I don't know.