There is a lot of speculation and blame in the responses here. Went to the Youtube page and the poster was the diver actively trying to help the panicked diver. He says in answer to a question in the comments that they were all separated from the dive leader due to poor visibility, so there's no point in lambasting him as an incompetent dive leader. He stepped up to try to keep the rest of the group together.
There can be a any number of factors that ultimately contribute to her panic response. In another setting she might have been a competent and capable diver, though I doubt she was very experienced. There are a lot of vacation divers who have only ever dove in clear, warm tropical locations with no current and fantastic visibility. Maybe she was one of them.
Others have said she was on holiday at the time of this dive. If she was inexperienced (likely), and if she was using rental gear that was unfamiliar (very possible), and in conditions that were also unfamiliar (cold, low vis) - her stress level was going to be high from the outset. Then add being separated from the dive leader, and fear of a big shark coming out of the gloom (it IS South Africa, after all), and the stress and fear start to ratchet up. It wouldn't take much more to push her into panic. Whether she ran out of air, took a CO2 hit from hyperventilating, or simply realized that others were going up and she wasn't - something pushed her past her tipping point.
I remember my very first open-water experience breathing through a rented regulator - I began hyperventilating and "overbreathing" my reg - and my reptilian brain took over when I perceived that I "couldn't get enough air" out of it. I was convinced that it was malfunctioning. I wanted it out of my mouth and to be on the surface as fast as possible. It's not a rational conclusion. Fortunately, I didn't spit it out and zip to the surface, but I DID end the dive and went back to the boat immediately and decided my diving days were over. (I got better...)
The hand waving and bicycle kicking are outward manifestations of her mental status - she's not thinking about what she's doing here. Others have commented that she failed to respond with a thumb's-up to acknowledge that they were going to begin their ascent. That's another big red flag - as if the flailing arms and ineffective kicking aren't signs enough. I am certain that getting in her face early, you would see huge eyes, and have a hard time getting eye-contact with her. This is the time to grab her and help sort out whatever problem she's dealing with.
Rejecting the reg AND the mask, plus trying to rip off the hood at the surface all are indications that she's in complete panic - no rational thought - just; get this stuff away from my face so I can breathe!
The signs of extreme stress are readily apparent early-on to anyone who is paying attention. As disturbing as it is to watch, it is a good lesson to show every diver what to watch for in your buddy so you can help prevent a disaster. Once the tipping point is reached it is very hard to execute an effective intervention.
If you haven't done it - take a 'stress and rescue' class. Best class since OW.
