My rear caliper comes loose

Hello,

First make sure that the bolts are torqued at the correct specification.
Those calipers slide bolts needs to be at 27 ft. lb.
Also make sure that the threads are in good shape.
If all of those checks out, it will be a good idea to use thread lock to prevent the bolt from coming out due to vibration.

https://www.2carpros.com/articles/brake-caliper-replacement

I have attached diagrams for your reference.

Please let me know if you have any questions.
Thank you.