Mommy, why am I a Tiefling?

A Quora user asked the question:
In DND, can a Tiefling be born from two human parents?
My answer to any question along these lines is almost always going to be:

Yes!

(setting norms permitting)

To find out why mummy and daddy are human and you're not, pick or roll 1d20:

1. Thauma-genetic manipulation at the fertility clinic.
2. Surrogate birth.
3. Cuckoo demon left an extra bun in their oven while no one was looking. Your twin sibling looks perfectly human though.
4. Parents made a pact with the Infernal powers, dedicating their next child to the Grand & Decadent Cause. (ANGSTY TEEN TIEFLING: Jeez! Lay off mom and dad! I don't wanna do your stupid dark destiny plans!)
5. One of your parents used to be an infernal pact warlock before you were born. They turned from that path a long time ago, repented, and tried to make amends for the things they did during those times. But a part of the pact they'd overlooked ensured their first born would be a Tiefling. So here you are. Sorry honey.
6. The car you were conceived in was situated in a lower planar energy nexus at the time. Had it been a couple parking spots to the left, you would have shadow-planar traits instead.
7. The nurse spoke up, "Leave this one alone!" She could tell right away, you were bad to the bone. (approximately 35% of all infants identified as bad to the bone at birth develop Tief-Ling Syndrome prior to the onset of puberty)
8. Your parents ran afoul of some sort of dubious magician who put the whammy on you in utero.
9. One of your fairy godmothers really donked up while giving you her blessing. Tiefling is kind of a blessing, right?
10. Your parents biological child was just a real jerk, and down in the Abyss you were just too nice to be a demon. So a trade was arranged. Everyone likes it better this way. Forget your biological parents, these humans are your real parents.
11. You were a child born during an eclipse. This kind of thing just happens sometimes in cases like yours.
12. During the World Rending War one of the ancient nations summoned the vast, bloated avatar of an outer god, which their opponents managed to slay at great cost. But its mountain-sized corpse, though fallen, would not decay. During the decades of famine which followed the war, your ancestors survived by mining into the carcass for still-fresh meat to sustain themselves. But eating strange foods has its consequences. Even now, hundreds of years later, children descended from such stock occasionally manifest the Tiefling trait as a result.
13. Nah! It wasn't ancient ancestors. You're supposed to give a pregnant woman whatever she craves, right? Well how were we to know her cravings for fried demon liver would make you end up like this!?
14. If your parents both have the same dream before you're born, it shapes something about who you are and what you'll be. Your poor parents ran afoul of dream demons and their dreams were awful in the months before you arrived.
15. This is what happens when a child gestates in the presence of excessive heavy metal.
16. I mean a lot of kids go through a “tiefling phase", right? Parents just hoping you'll grow out of it and become a responsible citizen at some point.
17. Being a elf or dwarf, etc. has nothing to do with ethnicity or genetics. Like all kids you go through a “Choosing Ceremony” at some point and are magically infused with the archetype you feel most connected to.
18. Did I say “Choosing Ceremony”? Sorry, I meant you spin a cocoon and emerge as your adult metamorphosized form. Dang, Tiefling!? You wanted to be a Dragonborn.
19. You merged with a demonoid symbiote. Instant Tiefling! Maybe it means you're chosen to be a hero or something?
20. You were just an ordinary kid dressing up as your favorite anime character Ty Flynn for cosplay day at the amusement park. Of course your friends insisted on riding that stupid Dragons & Dungeons ride and now you're stuck like this!