TBH the most ridiculous trope seems to be "mom sees sons dick and cannot resist her primal urges."
Nobody actually thinks like this and while I get the appeal, I honestly just laugh at that dialogue most times.
To be fair, there aren't a gigantic number of real-world plausible explanations for "mom starts humping son" scenarios:
1. Mom's actually a closet pedo/hebe/ephebo-phile, and son's clumsy attentions finally brought out her inner perv.
2. Genetic Sexual Attraction--son's been away from her so long, and away during his pubescence, so when they get together, the Westermarck effect doesn't apply, and she feels a strong sexual attraction to him.
3. Dad has neglected Mom for years, or she's been divorced/widowed and lonely for years, plus has some underlying attachment issues to son, and son starts showing her attention, and stuff gets out of hand quickly.
4. Son is unusually confident, attractive and manipulative, Mom is unusually submissive and in need of attention...
5. Son is a manipulative, blackmailing, abusive rapist monster who finds a variety of way to gain control over the women in his family.
6. Son is an attractive young adult(say, 25). Mom is an attractive middle aged woman(say, 44). They get together in unusual circumstances and things just happen. No manipulation, no "corruption", no blackmail, no weird impulses(well, except...). A true love story, in a manner of speaking.
Scenarios 1 and 5 are quite dark. 3-4 are middling scenarios where there's some moral ambiguity. Scenario 2 and 6 are the "too good to be true" "innocent incest" scenarios. The problem with 2 and 6 are that they set up scenarios where the family relationship is limited in some ways--they haven't seen each other in years, or he's old enough to have a life that doesn't center around living with his mom.
Beyond those reasons, you then get into the realm of fantasy or magic realism:
7. MC discovers a means of influencing the thoughts, feelings, memories and actions of others--a magic ring; hypnosis; a magic spell(s); a faerie/succubus helps them; actual mental powers; special pheromones; chemicals to put in their drinks; etc.
8. For whatever reasons, small acts and steps are adequate to slowly "corrupt" family members into full blown incestuous acts
9. The entire family is a hotbed of barely suppressed incestuous thoughts and feelings
10. Variations on and combinations of all of the above--both legit and fantastic