What leads Pip to think that Miss Havisham is his secret benefactor?

All of the available evidence appears to suggest that Miss Havisham is indeed Pip's benefactor. For one thing, she's incredibly rich, and Pip doesn't know anyone else with enough money to be able to make such a generous bequest. Pip comes from a very humble background, and Miss Havisham is the only person he's ever met who could be classed as wealthy. So it's not surprising that he should think that the eccentric old lady is his benefactor.
It's also notable that Pip is informed of his sudden good fortune by Jaggers, Miss Havisham's attorney. Pip has seen him on a number of occasions at Satis House, so when Jaggers pays Pip a visit to tell him about his "great expectations," Pip immediately jumps to the conclusion that Miss Havisham must have something to do with it.