What causes a pituitary tumor that produces excess amounts of growth hormone?

Dumb luck: There are rare genetic syndromes, but most strike at random. It is nothing you or anybody else did. If you're a youngster or teen, it'll make you taller and make your face "unconventional" -- your choice; if you've reached your full growth, you owe it to yourself to have it managed by whatever means is best -- seek a couple of opinions. Good luck.