Such a sad story, like you all I've been following it closely as it really is bizarre.
Can I just emphasise as others have, a lot of you are SEVERELY underestimating falling unexpectedly into cold water. It was 2 degrees celsius at night, so the water was probably close to freezing. You fall in that fully clothed, you can't scream, you can't move and you will sink. In addition, this is not a pool, there are weeds, undergrowth and currents to contend with.
I live in London, and people regularly fall into the canals and die all the time (which are actually very shallow, but full of undergrowth & junk) - in fact a young fit man, only 30 years old, fell off his canal boat, simply misplaced his step, 100m from where I live and never came back up - and this is with people seeing it occur!
It took the police and rescue days to find him. Eventually, he was found at the bottom of the weir only 15/20 metres away.
Again, you have no chance when falling into freezing water wearing winter clothing/boots/jackets etc.
To add to this, I've actually experienced this myself supervised in a pool for a life-saving course in New Zealand. I went in wearing jeans, sneakers and a shirt - it's absolutely terrifying, and that was supervised and knowing what was about to happen.
My theory is the call/meeting was coming to an end, it was boring small talk.
She put the phone on speaker, and placed it beside her.
Perhaps she threw a ball or the dog was distracted and about to go into the water.
Knowing she was about to leave, and didn't want a wet dog in the car, she walked over to the dog to attach the harness.
As others have said, she was wearing essentially a sleeping bag.
As she has knelt down to attach the harness she's lost her balance and fallen in.
The shock has set in, and her clothes have weighed her down, pulling her to the bottom.
RIP