Sometimes crying "straw man!" is a method of avoiding
debate.
It is true that the poor debate tactic of erecting a straw man does exist and is often used. Unfortunately, accusing someone of straw man is also a powerful tool to shut up those to whom you do not wish to listen.
Being accused of "straw manning" is stigmatizing. To those in rhetorical circles, a debater who has been decried for use of a straw man can become something of an outcast (at least for the duration of the debate). As such, it can be a last-ditch effort of someone losing a debate to cry "straw man!".
The burden of proof that they are not, in fact, using a straw man then falls to the accused. This process can be highly digressive, which fulfills the accuser's intention: to deflect attention from their shaky position.
Again, this is not to say that it is invalid to point out the use of a straw man. But it should be recognized that accusing someone of "straw-manning" can be overly damning. Especially if the accused was not intentionally erecting a straw man.