Why would previously frozen steel cut oats separate, where just-cooked version doesn't?

Oats (like other starches) swell and absorb water when heated. Cooling them causes the molecules to change their shape and realign to form a different sort of gel, undergoing a process known as retrogradation. When starches are frozen, this network of molecules undergoes a lot of damage, due to the combination of ice crystals forming (and disrupting the networks) along with general thermal expansions and contractions. As the network of starches contracts with temperature, it also tends toward syneresis, the technical term for what happens when liquid is expelled from a gel. (Essentially the starches contract and "squeeze" the water they absorb out, kind of like a sponge that took in liquid and then is squeezed.)

Anyhow, starches are somewhat fragile when they're heated and absorb their maximum amount of water. When they undergo wide temperature shifts combined with expanding and then melting ice crystals, the starch networks can be permanently damaged so they can't hold as much liquid as before. Sometimes reheating can help reorient the starch molecules and allow them to absorb a bit more water again, but after freezing you will likely have some permanently damaged starches (leading to the well-known "grainy" texture that happens to many starchy sauces when they are frozen and then thawed).