I believe this is a type of stimulation resulting in a massive release of the neurotransmitter oxytocin, which could overload receptor sites, causing euphoria, similar to O but cleaner and no mess to clean up as with ejaculation. I do not see oxytocin as a sexual neurochemical. Though a heigtened sense of pleasure can be reached, i highly doubt it would be sexual. Still sounds tasty, doesn't it?
The nipples are erroneous zones, even for men. Touching, pinching, rubbing, clipping (with sex toys designed for this) can lead to sexual arousal, erection, and orgasm for many men. Like masturbation, it's a self-stimulation for sexual arousal purposes. It can also be stimulating with a partner's contribution manually or orally. There may be several men whose nipples aren't so sensitive and they'd probably not get it, experientially speaking. But, it's a real phenomena, and it's natural for many men who experience more or less sensitivity along a spectrum.