Sourcetree is just a wrapper for Git. There is a good chance that your repository is missing (or not configured) ".gitignore" file. The most common occurrence that I see is IDE specific files and folders like [.vs, .code, .ideal] are not include in the ".gitignore". So if you just open the project to look at code, AND made no changes yourself, your IDE could still make changes to setting or config files in the repo. Therefore Git and Sourcetree will see a difference and will ask you to commit.
There are some complications like if the ".gitconfig" is updated then you will have to update the cached (tracked) files so that Git and stop tracking it.
I would need more specifics to be able to help more.
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