Why does honey prevent porridge congealing?
Honey is hygroscopic - meaning it has the ability to absorb water. Even if you covered your porridge after adding the honey, there's still enough moisture in the container for the honey to absorb. 

You don't say whether your honey is from a local beekeeper or heat-treated store honey. The more honey is heated, the more natural enzymes found in honey are destroyed. One of the enzymes in honey is distaste (amylase). It has the property of hydrolyzing starch (breaking starch down into sugars). I can't state for a certainty as I'm not a food scientist but I'd guess the additional sugars might be liquid. But even if not, it would mean less cooked starch, hence less gelling ability of the porridge.