Trump even tried later to distance himself from it. He said he didn't like them chanting that, and he began talking very quickly to stop them. Which he didn't at all.
If you see that bit of the rally (I'm sure it'll be on YouTube), while the crowd is chanting 'send her back!', Trump is standing with his chin out. This is sort of a tell with Trump. At his acceptance speech at the convention, and again in his inaugural speech, Trump would say something tough-sounding, about American Carnage or whatever, then while the crowd cheered, he'd strut around with his chin out like that. You know who else used to strut around like that during 'spontaneous' applause? Mussolini!