A verb that already contains a direction does not take a preposition repeating it.

The 're-' in return, revert, repeat and recall means 'back'. Adding 'back' says it twice, and the examiner scores the second one as an error rather than as emphasis.

Except

'Go back' and 'come back' are correct — neither verb contains the direction, so 'back' is doing real work.

He returned to the office at four.

return already means 'come back'

He returned back to the office at four.

'back' repeats the 're-'

Where it bites

'Return back', 'revert back', 'repeat again' and 'recall back' are four separate planted errors drawn from one rule. Spot the 're-' and the redundancy names itself.