When it can be done with an incline, such sleeping is done by some to try to keep blood flow toward the head better than it would supposedly otherwise be or to try to deal with lower extremity swelling or clot formation/clot-related risk. Don’t ask me whether such sleep ritual on a flight makes sense or not, but some people have sleep rituals that can come across as eccentric when examined closely.

On some flights, there have been people sleeping in weird positions in economy class at times too. Things like sleeping in a backward facing position by using the seatback in front of them as a backrest while sleeping. Good luck trying to recline the seat when the passenger behind has your seat as a headboard of sort.