What generally causes pink mold?

Not a mold. Pink "mold" is probably a bacteria, Serratia Marcesans. The Washington Post article may help. https://www.washingtonpost.com/lifestyle/home/why-did-my-grout-turn-pink/2017/03/31/0bb5e8c8-0fea-11e7-9b0d-d27c98455440_story.html?utm_term=.2e479890b1f1.