Testers throwing cases back to me after passing them months previously

I seem to be having a lot of my cases thrown back at me after passing QA months earlier.

Quite often I'm finding that it is work from other cases which is impacting the results of my work.

I.e. I fix A, fix A is tests and passes, someone else fixes B months later and that's passed too… But then case A is dug up out of it's grave and thrown back at me as a result of changes in B.

What am I doing wrong? What processes should be implemented to prevent this?