It’s atomic force. I know that because I worked as a scientist in physics of solids and we used vacuum equipment in our laboratory.
Parts are typically made of none-rusting materials (like stainless steel) and are cleaned thoroughly before sending them to the outer of space. Space vacuum removes rest of the air and residues from all surfaces including fasteners. When 2 surfaces that are so clean come into close contact (the is no lubricant to separate them), they stick to each other by atomic force.
On the top of that if this contact lasts for a long time, atomic diffusion kicks (this takes years) in and parts are getting literally fused together by the process of atomic diffusion.
The remedy we used in our particle accelerator was special carbon-based lubricant that works in vacuum and does not give off gas