Panel 1: We start things off with Farley sniffing at a bone. Since he's a dog, he doesn't know that there's a string tied to it or what to expect when he sees something like that.
Panel 2: The tone is set when the bone is yanked away as part of a mean-spirited stunt.
Panel 3: As Farley walks down the hall, we see Mike lying in wait for him.
Panel 4: He yells AH-HAH!!! and subjects Farley to a flying tackle.
Panel 5: He gets Farley into a Big Ball Of Harmless Mayhem because that wouldn't get his ass bitten in this strip.
Panel 6: He puts a pillow-case over Farley's head in order to confuse and frighten him.
Panel 7: He puts a sieve over Farley's food to be a jerk.
Panel 8: Someone off-panel has finally noticed what Michael is doing (trying to make him think that a dog hand-puppet is trying to eat his food....and succeeding for some reason) and yells his name in order to make him stop screwing around.
Panel 9: John asks Mike why he's tormenting Farley.
Panel 10: John is gobsmacked when Mike tells him that he's tormenting Farley because he can't find Liz.
Summary: The reason that this surprises John is that since he only ever gets Elly's side of his kids' life story, he has no real idea that she's stupidly pitting them against one another lest they team up to destroy her. Also, he doesn't quite know that Mike has made a life and living of tormenting Liz for forcing him to share things like he's a nobody that everyone despises.