I can't know for sure, because we're all different in some ways, but my thoughts are these:
You say that DD1 was walking up and down reciting a book. To the NT (neurotypical, i.e. normal) way of thinking, that means we're happy. Not exactly. If we're doing something very repetitive, it tends to be because we're 'out of spoons' - in other words that something else has already happened that we found too exhausting. So we repeat ourselves over and over again and do set movements as it calms us down.
Along comes DD2 who bumps into her (which feels a bit like a major slap to us as we tend to be hypersensitive to unexpected touch), so we're already as shocked as if you'd just come over and hit us. Then DD2 make a noise that sounds like a jumbo jet taking off just next to our ears, then mum starts moving very fast and closing in on us faster than our brains can take it all in, then maybe mum looks right at us so there's the scary eye contact thing going on even if mum meant it to be kind, and she's saying something but by now our brains are Totally Out of Spoons and way, way beyond remembering how to calm ourselves down again or understanding what you're saying. So we wail, and we hate wailing as it's loud and awful, and it makes us wail even more. Yes, it's silly to do something to ourselves that we hate, but small children haven't worked out how to help themselves properly yet. Worse still, the wailing makes mum talk to us more and maybe even hug us, which is more sensory overload. All good things if you're NT, not so good if you're many children with an ASD.
Things that might just work: Watch out for when DD1 is doing something really repetetive. She might be far closer to screaming than you think, and may need quiet and rest more than you realise. If DD2 bumps into her and falls, keep all movements slow if you can, voice low, no eye contact with DD1, get her environment back to really quiet and repetitive as soon as possible. I find a thick duvet to put round us works for some of us, because it's so overwhelming that we can blank out the scary thing that just happened. Might not work for DD1 though. Other than that, make sure she's safe, quietly put her favourite repetitive things within reach, wait it out, and let her rest afterwards. Then the next day maybe work through a social story with her - pictures - so she can see what you will do if DD2 bumps into her and she can anticipate it next time. If she's good at counting, maybe teach her to count out loud when something scary happens, as it's repetitive and predictable and we can do it anywhere. It works for me (though in my head, not out loud )