Peak drama for chess. Hans cheated in the past online, he admitted it following Magnus' initial refusal to play him, but he recently received another ban from that site because he lied about the extent to which he cheated. So, either Magus has worked a probability that he thinks indicates cheating, but without the definite proof to publicly expose him, or Hans got tips from people close to Magnus about how he would play heading into the match, Magnus recognized this and refuses to play him from that point on.
I'm leaning towards the cheating side, because Magnus refuses to comment on it, he noted the possibility of being in trouble for commenting on the situation, and any reasonable person knows how foolish it is to accuse someone of cheating without definite proof. Without that proof, you are always working on "it's likely he cheated" but that obviously won't be sufficient for the chess community, especially when some think Magnus' refusal to play Hans is simply bad sportsmanship towards a player that has bested him, as opposed to a refusal to entertain a cheat.