This time Hagrid, unlike usual, hears something he should not have heard.

Hagrid overhears a private discussion where Dumbledore and Molly are plotting to take the Potter fortune, granted he only hears Dumbledore complaining that he can only embezzle funds from Harry Potter's trust vault, due to the goblins not giving him access to the family vaults, despite worming his way into becoming Harry's magical guardian.

Hagrid knows that Dumbledore has his reasons for stealing from young Harry, probably good ones too, but what is the boy going to do without his money? No, he thought, it simply would not do to have little Harry being raised by the Dursleys, which he vehemently yet silently disagreed with in the first place, only to arrive in the wizarding world broke.

Hagrid has been selling Acromantula venom to Shyverwretch's Venoms & Poisons in Knockturn Alley, Acromantula Silk to Twillfit & Tattings & Madam Malkin, and Unicorn Hairs to Ollivander's for nigh on 50 years. Not enough to crash the market for those ingredients, but enough to have saved up one hell of a fortune, and he lives a pretty simple life.

So Hagrid starts frequently donating money to Harry Potter's vaults anonymously under various names, including Stubby Boardman.

Cue the next decade and a half of Dumbledore tearing his hair out in frustration. I mean, Dumbledore took out Grindelwald, he might have waited to do so until months after World War 2 was over and Grindelwald's army was defeated and until Grindelwald himself was severely magically drained after defeating 10 wizards nearing his level of power, but still he had never just been given any money for the accomplishment, and yet here's Harry being donated gold left and right for defeating Voldemort.

For the sake of argument, Hagrid being a half-giant is magically-resistant and that makes performing active Legilimency on him much more magically intensive than it's worth, and passive Legilimency simply doesn't work on him.