1). Mason “acquired” Pym particles and decides to use it to his advantage, after giving Natasha the trailer to hide out in, he comes back to perve on Natasha by shrinking himself.
2). The Red Room uses shrinking to create tiny assassins, they get sent to the Budapest hide out to retrieve the antidote undetected from Natasha and Yelena, but given there size it turns out to be a bad idea.
3). Melina, who doesn’t want anything to do with her fake “family” decides to shrink them all.
4). After the whole team gets captured, Dreykov decides to shrink them all and orders the red room assassins (including Taskmaster) to have fun with them.