Maybe they're tired from eating all the other capybaras, they leave that one alone because he always leads a new herd to the beach. Heh.
There really is something to their latent ability of mesmerizing chill I think, the acceptance they get from other animals is pretty insane. Can't say I know of another animal that can pull it that off, cleaning fish on a reef maybe? I've seen pics of them chilling with crocs before, though I suspect like most things there are limits. The clip I posted is probably the most pronounced example I've come across so far, but I still don't expect we'll see pics of a capy spooning with a bloody big jaguar.