Don't drop or bump the tranny too hard while it's out of the car! It looks like you had the tranny in first, bumped the tranny on the bellhousing end which slid the 3-4 fork up. That allowed the selector to get behind the 3-4 fork. More bumping shifted the rest of the forks and locked it in position.

Leave the trans in neutral. Don't set it down so hard on it's end. Row through all the gears of EVERY trans before you install it to make sure nothing is F-ed.