In Chapter 1, everything is still a game to the boys. They have just found out that they are on the island alone, with no adults. They are realizing that they can pretty much just do what they want. Their tipping the rock into the canopy of the jungle is an expression of their excitment and exuberance. It is something that they do simply to show that they can.
There was no reason for them to tip the rock into the jungle. Jack pushed it and it moved a bit and then they all saw it as a challenge. It was just something they wanted to do to prove that they could do it. Pushing the rock off the cliff gave them a moment of exhiliration and it just seemed cool to them -- so much so that they couldn't leave for 5 minutes they were so excited.
So, in this chapter, you have a bunch of boys who have just found out that they are free from all rules and all adults. They want to have fun and challenge themselves. Pushing this rock off into the canopy is an example of this.