My loaches do it occasionaly. Its just a simple territorial fight. I find it quite cute and funny. They should erect thier fins, twitch a lot and thier black bars may or may not turn purple-ish.
I have three Clown Loaches that tussle for the prime spot in a log. When they get real excited you can actually hear them "popping" at each other. There is one in particular that "pops" at snails and then consumes them. It does this to glass shrimp and appears to stun the shrimp for a moment. It is very interesting and have not seen it mentioned in literature yet.