The feature you're referring to is called Spring Loaded Folders. Folders should only spring open if you hover (or pause) over a folder for a second or so (depending on your settings).
In other words, if you drag and drop files from your Desktop into Folder A you would let go of the mouse button as soon as your pointer is over Folder A, rather than pausing over it for too long.
You can adjust your settings as follows:
- Go to System Preferences > Accessibility
- In the left-hand sidebar, find and select the Mouse & Trackpad option.
- Now use the slider to adjust the Spring-loaded delay time.
NOTE: If you wish to disable the Spring-loaded folder feature, you can remove the checkmark next to the slider.