If an on-going index build yields its locks after initiating a bulk insert (which is initialized here), it still holds onto the write lock on the index table at the WiredTiger level. If a dbStats command comes in, it will take collection level MODE_IS lock and attempt to acquire a read_lock for the ident the index build is currently writing to (but cannot since IndexBuild_1 holds the exclusive lock on that ident). (In (collection_impl.cpp) we iterate through the unfinished indexes and that is how we can see the in-progress index table).
  

The problem arises when another operation comes in and prevents IndexBuild_1 from re-acquiring its lock, like another index build that enqueues a collection MODE_X lock. These events can produce a deadlock in the system represented by:

Original explanation by Suganthi here