We know pretty directly how the corruption came about, being a malignant bit of your world fueled by the enherent evil of a terraria world, it's an evil part of the world like cancer.
But how does the crimson happen?
I would like to posit that the crimson is a prion infection, spread from the rotting nervous tissue of the moon lord.
If you're unaware, a prion is a fucked up protein that, apon coming into contact with other proteins turns them into prions as well. We know shockingly little about them but they seem to form in dying nervous tissue, and are contracted by living creatures when that tissue is consumed.
The crimson chasms are literally massive calcified nerve cells from the moon lord and the prions from them convert your world into more of itself.
by KarmaticDeer