Why would intellij be decompiling my own classes

I have a gradle project with a bunch of sub-projects.

When running a single test (in IDE) - apart from the delay of building absolutely everything - it then spends minutes decompiling my own classes, locking up the IDE and pausing test execution.   

I see a progress popup spining through all my familiar class names….   “decompiling”

It then reloads the source in open editor tabs. 

To help me fix this, can anyone tell me the scenarios whereby IntelliJ IDEA is required to decompile local classes?