"It would certainly be interesting if we could know exactly what is going on in some late transitioning cases. Is it sexually motivated?"
Genuine question: are you new to this topic?
Seriously, there is a wealth of research, first person accounts and more evidence on the likes of reddit than you can shake a stick at.
"Can that genuinely turn some people trans?"
As in, "Can that genuinely make some people believe that they have actually changed sex?"
Possibly but glibly calling that state of mind "trans" is part of the problem. If someone genuinely believes that they have changed sex that can be a symptom of psychosis, ie. the person is psychotic and that is just one of the symptoms they might evidence.
For example, Andrew Burns / Tiffany Scott (aka Obi Wan Kenobi/Mighty Almighty)
https://transcrimeuk.com/2017/10/31/andrew-burns-tiffany-scott/