Cleveland Cavs -- Launch Internal Investigation ... Why Was Gatorade Poured Into Powerade Bottle?

The Cleveland Cavaliers tell TMZ Sports ... they have NO IDEA why a ball boy was serving players Gatorade masked in a Powerade bottle last night -- but they're gunning to find out why.

Twitter EXPLODED last night during the Knicks-Cavaliers game when a TV camera caught a Cavs ball boy making the switch behind the Knicks bench.

Now the Cavs -- who have a sponsorship deal with Gatorade -- tell us they're investigating why their official drink was poured into a competitor's bottle.

"We are looking into why the Powerade bottles were used. We're trying to figure out what happened."

The team made sure to clarify that the incident did not occur on their bench -- however. it was a Cavs employee who made the switch.

So what the hell happened? "That's what we're trying to figure out."