We spoke with Mark ... who tells us he was working out in a turf area in the outside area of his gym in Florida when Richie came out and started acting in a bizarre and erratic way.

Mark says Incognito was rapidly switching between exercises with no real plan and then "went to the boxing mannequin and tore it off its pedestal."

Richie then started cutting off Mark while he was jogging in the area ... and when Mark called him out, Richie pegged him with a tennis ball.

Mark says Richie grabbed a sled and "tried to run me over" and then grabbed a dumbbell and launched it at him.

That's when Mark called 911 -- and Richie accused him of working for the government.

Cops arrived and put Incognito in handcuffs before placing him on an involuntary psychiatric hold.