what actually can cause adult strabismus?!

No, eyestrain doesn't cause strabismus. (It might re-aggravate one you had as a kid, though.)
Things that cause strabismus in adults: a decompensating phoria (i.e. a slight misalignment getting worse as you age and your muscle tension deteriorates), a stroke or similar brain problems, head trauma and/or skull fractures, out-of-control MS or thyroid problems... a whole list of stuff that typically is super-obvious.
That being said, eyestrain isn't a good thing, and your friendly neighbourhood optometrist should be able to set you up with a solution.

I'm an optometrist.

No, excessive computer use cannot cause strabismus, strabismus is present from birth. Maybe a stroke could cause adult strabismus.

The excessive computer use can cause is eye strain.

Good luck

Eye care specialist.

No, it would take more than that. A stroke is a good example.