Going on hunch as big breaches such as these have been inside jobs (e.g., Walkers, Hanssen).
Knowingly or unknowingly, trump has made numerous leadership selections in his administration and some were very questionable.
Not outside the scope of reality that foreign intelligence agencies could have received support in getting keys/access to these agencies from one of these appointees (e.g., placing devices to listen to communications, providing access to encryption devices).
This was a major breach.