My guess is the Code Completion plugin access wxWidgets things (building the symbol browser tree for example) in its worker threads and since wxWidgets makes no thread safe guarantees this kind of stuff will happen.
I don't know how the Code Completion works but due to the numbers of thread related errors we are encountering it looks like not enough time was spent during its design phase to carefully define where it interfaces with the rest of CB and then provide thread safe mechanisms of communications.