Jem was not too happy about Scout's reaction to his story about Hot Steams. Dill had never heard the term before, so Jem explained that they were a type of spirit that had not yet reached heaven. Scout discounted the tale, claiming that "Calpurnia says that's nigger-talk." So, Jem determined to pay Scout back quickly. When she demanded to be first when they rolled the tire, Jem pushed her roughly and "with all the force in his body," and she landed on the steps of the Radley house.
The surprises left in the knothole of the Radley oak tree ended abruptly when Mr. Radley cemented the niche. He apparently had discovered that Boo was leaving gifts for the children and disapproved of his son's actions.
Jem gets even with Scout for contradicting him in Chapter 4. The kids decide to play by rolling in a big old tire. Scout goes first and Jem pushes her. But he is angry so he pushes her really hard and she rolls all the way up against the Radleys' front porch.
The reason that there are no more surprises in the tree is that Mr. Nathan Radley plugs the hole up with cement. This happens in Chapter 7. Mr. Radley tells Jem that he has plugged the hole because the tree was sick.
Jem gets even with Scout's dissent about the "hot steams"--which she has dismissed as mere folk lore--through an act of physical aggression. While the children are playing with an old tire, they get the brilliant idea that each of them should climb inside of it and roll away. Scout insists upon being the first to climb inside, and Jem seizes his opportunity for revenge by giving her an extra hard push, which causes her to roll all of the way up to the Radley's house.
To answer your other question, it is determined that there will be no more surprises in the Radleys' tree after Mr. Radley plugs the secret hole with cement. While Radley claims that he did this because the tree is sick, it seems he disapproves of his son's habit of leaving gifts in the hollow for the neighborhood kids.