So many possibilities. Perhaps she slipped peering over a ledge, perhaps she opted for the lake trail thinking that it would be shorter - perhaps thinking she could collect some water along the way. My experience is that temperatures are colder near mountain lakes and rivers.
Mountain lakes are typically extremely cold, even during Summer. Drowning occurs in minutes. Wearing a pack, she wouldn't be able to get out of the water in time even if she was able to release the pack.
Includes drone footage of the area