Apparently Treach has some questions about whether the Geto Boys' Bushwick Bill is down with O.P.P. ... so he pushed him off the edge of the stage they were sharing.

We're kidding about the reason, but Treach and Bill did take a tumble during Naughty by Nature's set of their "I Love the 90s" stop in Houston. Bill was rapping his verse to "My Mind Playing Tricks on Me," and Treach decided to join him, but tripped on his way.

Clearly, Treach believes the more the merrier -- 'cause he wasn't going down alone!

It's all good. Bill was smiling afterward, but we're guessing he didn't stick around for "Hip Hop Hooray."