I believe the issue stems from a faulty GUI click layout in the Sourcetree stash popup.

Here, the stash option "all" is activated involuntarily when the user miss-clicks above the Ok or Cancel button leading to unwanted all stashes without any warning. I would assume the click should only work, when clicking on the actual checkbox or checkbox label and not the entire row. This is the case for the first option "Staged files / Selected files" but not the other options. For those the entire row is clickable. This leads to a big issue in one of my current projects.

I'm currently working on a React project with a large folder "node_modules" inside the  git tracked project folder. Involuntarily stashing with option "all" means stashing 5.5k files and >600 sub-folders. There is no warning or anything when clicking Ok. What I don't understand here is, why the existing .gitignore is not consulted, because my expectation of the "all" option would be to store all files I care about and that are relevant to my code. So basically all files shown in the "Unstaged files" section of Sourcetree and not all the changed/untracked files in the project. There is no need to stash files I specifically ignore.