I believe it is usually when they want to do it another way and are just trying to convince the professionals to change their thoughts, right or wrong.

The same happens to me when I'm at work and another tradesman will ask with an idea already in mind as to what should be done. After I give the advice he/she will debate their idea expecting me to change my initial advice. Except for Jerry...Jerry's cool and always does just as I say ( and he's sitting behind me right now).

I have always called it biting the hand that feeds you. The DIY'er comes here, asks a question, a pro answers it, then the OP wants to argue with the pro. What's to argue, you asked a pro a question, he answered it to the best of his knowledge from the info given by the OP. Now the OP want's to debate the answer. Why ask in the first place if he/she already knew the answer (in his mind). It's happened to me at least a dozen times.