Lamar Odom -- Khloe Questions Set Off NBA Star

It's now clear why Lamar Odom flipped out on a paparazzo in L.A. yesterday -- he was pissed that the snapper repeatedly hammered him about a report he cheated on Khloe Kardashian.

The confrontation was all captured on video as the photog -- not a TMZ guy -- rolled up to Lamar's car and asked if he stepped out on Khloe with a stripper.

At first, Lamar calmly answered the question -- saying, "I would never cheat on my wife, that's why I wear my wedding ring."

But things quickly turned -- when Lamar asked the photog for his name and then said, "I'm gonna step out the car and I'm gonna talk to you."

The video cuts ... and when it turns back on, Lamar is in the middle of the street with the photog's gear ... and eventually puts it in his trunk.

The photog pleads for his stuff back -- Lamar replies, "Shut up."