The author of the sentence wants to stress the verb "zanken".
In German, you can generally stress a part of the sentence by moving it to first position. On the other hand, in a main clause, the finite verb must always come in second position.
In future tense, present/past perfect, and passive voice that's not a problem, since there is always an auxiliary verb (werden, sein, haben); so the infinitive or participle can take the stressed first position and the finite form of the auxiliary verb takes the second position.
In present and past tense active, however, there is no auxiliary verb.
The problem is solved by using the finite form of "tun" in second position as an "auxiliary auxiliary" verb together with the infinitive of the main verb in the stressed first position. This is colloquial style, though.