Kate Upton & Maksim Chmerkovskiy -- WHAT THE HELL Is Going On Here?

Somebody's got some explaining to do ... because model Kate Upton and "Dancing with the Stars" pro Maksim Chmerkovskiy ... the most random two people ever ... left a restaurant together Monday night in NYC.

There were reports the two were spotted canoodling inside ... but sources close to both camps tell us, they're not together. Not even close. They're just friends.