The Maple Leafs elected to have an arbitrator decide Jonathan Bernier's salary for next season. This process is adversarial and puts the outcome in the hands of a third party.
Why did the Leafs choose salary arbitration?
The Leafs filed for salary arbitration against Bernier almost as quickly as it is allowed. Bernier is a Restricted Free Agent as defined by the Collective Bargaining Agreement (CBA).
We don't know how negotiations proceeded through the end of the season, but obviously both sides were far enough apart on term, money, or both, that the Leafs saw a risk that they would not otherwise get a deal done.