General wheel knowledge with Boeing commercials.
If it was just the tyre then a wheel flange failure would be suspect ( nearly unheard of since detachable flange wheels have been phased out for modern pax aircraft); if whole wheel and tyre then it looks very much it escaped just as the retraction braking cycle began (up line selection) and this happens to be the point at which the tenons would be aligned and the wheel could slip off ( same sequence for fitting a wheel unless you want to have a lot of fiddling to align each disc).
Don't know exact weight of the combo, but if a wheel change dolly is not available it is a job for two Mechanics (not the Hulk) to lift onto axle (may use a thread protector).
That particular wheel position, the rotation would have been in the direction of tightening the axle nut and so doubt it was just the locking bolt missing.
Info: I found a crack in a flange wheel (Visount circa 1966) using ultrasonice probe when older method was dye penetrant; commercial aircraft wheels are subject to NDT, as is the tie bolts, at regular tyre change intervals.