Detransitioning is when someone reverses their transition. This can occur socially, via returning back to one's deadname, old pronouns, and old social identity or way of dressing and otherwise presenting. Medical detransitioning consists of reversing surgeries or hormones with additional surgeries and hormones. Detransitioning is extremely rare.

Those who are still transgender.

Sometimes, those who are transgender feel a pressure to detransition due to external forces such as transphobia (harassment, discrimination, lack of community support), or finances (for example, being unable to continue paying for hormone treatments when not covered by insurance).

Sometimes people transition to a binary gender (ex. FTM or MTF) and then later detransition to a non-binary gender that makes them more comfortable.

Those who made a mistake.

Sometimes people question their gender identity and come to the conclusion that they are not transgender. There's nothing wrong with that. Usually they realize in the questioning stage.

Those who made a mistake and cannot take self-responsibility.

The tiniest minority of the already very small group of people who call themselves detransitioners are those who went through some kind of transition, and have taken it upon themselves to blame the entirety of the transgender community for their own mistake. Instead of recognizing that they were mistaken upon being transgender, they decide to blame and demonize an entire group of people. While the rate of regret for transgender surgeries is astronomically low, those who do tend to fall into this category.

3rd biennal EPATH Conference Inside Matters. On Law, Ethics and Religion
NIH - Abstract: A Survey Study of Surgeons’ Experience with Regret and/or Reversal of Gender-Confirmation Surgeries
NIH - Factors Leading to “Detransition” Among Transgender and Gender Diverse People in the United States: A Mixed-Methods Analysis
Reddit - r/actual_detrans
YouTube Video: Detransitioning Transgender
YouTube Video: Cuda: My Honest Opinion About Detransitioners | FTM MTF DESTRANSITION
YouTube Video: THE WORST TERF WANTS ME TO DETRANSITION (FTM TRANS) | NOAHFINNCE