I am not quite sure of his motivation, but Sam has started what can only be called "baby tipping". Perhaps he feels that Sean needs some tough love, maybe he is jealous or maybe he just thinks it is funny... whatever the case may be , "baby tipping" sightings have become a frequent happening around these parts.
So here is how it all goes down... literally. Sam goes up to Sean and positions himself behind him, gives him a kiss on the top of the head and than puts his hand in the middle of Sean's back and gently pushes until he topples over and does a face plant into the carpet. Quite pleased with himself (as if he has performed some great public service), Sam then moves on to other things. Sean in the mean time is left wailing on the floor in an almost frozen state with is face in the carpet and his butt in the air... as if he is locked in the sitting position when the tipping occurs. He feels betrayed and is not quite sure how to handle this sort of brotherly love. Sometimes Sam takes a full frontal approach and all I can say is some webbles DO fall down.
Over all Sam still remains quite affectionate towards Sean but this baby tipping thing seems to be more of an experiment in Physics than an act of malice. Gravity is often fascinating and cruel. It is almost as if Sam is testing him... sort of like a Zen master trying to explain the importance of balance through example. Practice and patience young Grasshopper.