Two people suddenly like/favorite everything, or almost everything, the other person posts while not showing publicly that they are close.
It means that there is a certain tension and excitement between the two. Liking is equivalent to paying attention to someone or giving someone a compliment/support in the offline life. It shows that you are interested in someone as a friend, colleague or as a potential partner. Sudden mutual liking usually means that two people will soon share a private inbox, eventually they might share a bed as well.
Usually this initial excitement about the other person disappears as suddenly as it appeared, but sometimes it turns into a proper public relationship or friendship.
“Have you noticed that Elena and Maggy SML a lot lately? I don’t think they even know each other in RL.”