[cypher]

It is just one of those things they do sometimes, has it been a recent thing? They can do it to protect their food stash so may start after a big clean out or something like that, sometimes they just seem to like things that way!
As long as the food isn't getting really wet I wouldn't worry, if you are concerned the food has got peed on a lot then you could just swap it for some fresh now & then, with any luck your ham will find a better place soon!