IMO, a large part of it is about perfectionism and fear - fear to the point of terror - of error, where "error" is normally defined as errors of commission, not omission.
Throwing out the expired tuna could be an error, because the tuna might not actually be bad. Throwing out that plastic cup could be an error; someone might want to use it. Anyway, it has a cartoon character on it; it could be collectible. That used envelope has a plastic window - putting it in paper recycling might be an error. Putting it in the trash might be an error. Ripping off the plastic window will take some paper with it and that won't be recycled and that's waste. Anyway, the address might be useful. Anyway, it might be useful as scrap paper.
All of these feared errors are errors of commission. The hoarder doesn't see that the errors of omission - the failure to throw out stuff that results in the unlivable home, the unhappy family, the marriage at risk, the children possibly lost to the state, the dust, the dirt, the illness, the risk of fire, the risk of death of oneself or one's loved ones - cause far, _far_ more harm than these errors that they fear.