The verb is generally in the second position in statements. Without derfor it would be «De er hjemme», but when derfor takes up the first position, the subject is moved after the verb.

This is called subject-verb inversion. The verb must always take the 2nd position of the clause. This is easily noted in sentences that begin with a location or time/ "I dag, regner det mye." (Today, it rained a lot.) Notice how the verb "regner", takes the spot before the subject "det".