Is "checking it down" in a tournament implicit collusion?

If you have one all-in player, and two other players who still have stacks left, is it 100% expected that the two "live" players simply check it down?

No, but it's pretty common. The presence of a player whose hand will always be shown down makes bluffing generally inadvisable, and consequently (though less obviously) shifts the hand strength needed for a value bet upward. Each of these makes bets less likely, and so tends to promote a checked-down finish.

When the players are involved in a place-paying tournament or supersatellite, and are near or in the money, that is compounded by the value of eliminating a player. Because both players benefit if either one eliminates the all-in player, bluffs usually have negative value, and the desirability of making value bets goes down even further. This is the situation where most players automatically check it down.

Failing to participate in this "implicit collusion" will draw scorn from some other players. The ones no longer involved in the pot still stand to benefit from elimination of the all-in player, so they also want to see it checked down, as that maximizes the chance of an elimination.