what is the major conflict in chapter 7?

There is of course the stress between Ralph and Jack. The parachute man as the inferred beast in the tree is apparent as well.

Check out this from GradeSaver,

In this chapter, Golding further develops the themes he introduced in "Beast From Air." The rift between Jack and Ralph becomes more intense as Ralph continues to remind Jack of his misguided priorities. The struggle in this chapter between the two characters again assumes political overtones, as the two engage in a power struggle for authority over the other boys. The concerns of Ralph and Jack were established in previous chapters: the former focuses on survival and escape while the latter focuses on hunting and self-gratification. In this chapter Golding examines the tactics that each uses to assert his authority. Jack uses his bravado to signify his strength and dominance, and he attempts to diminish Ralph in the eyes of the other boys by ridiculing him for his supposed cowardice. Ralph, on the other hand, is straightforward and direct. He challenges Jack's overblown self-confidence by honestly noting that Jack is wrongly motivated by hatred.

what events lead up to that?

Cara, read the above summary. Those are the events. If you want an event by event summary go to the same link and it is all there.