Yes, it is a staffing imbalance. Any base is possible, it all comes down to what's available and who moves where. Example: Let's say an FO in BOS leaves, creating a vacancy, and the only FO that puts in a bid for BOS is a new-hire still in training. It is possible the new-hire would be awarded BOS right out of the gate (assuming the company did not override it, and refuse to honor it). It's all a numbers game played with timing & seniority.

See this thread that I have been updating over the past few months. It might answer some of your questions.
Domiciles availability based on Seniority - Page 4 - Airline Pilot Central Forums (airlinepilotforums.com)