Until 6 months ago I never had any reservations canceled by the airline, but recently they do seem to have become way more proactive.

For example, I had an LAX -> NYC booked for Friday evening and one booked for the Saturday morning. I was in the process of calling to upgrade the Friday one, and cancel the Saturday one, when the agent told me the Friday one had already been canceled by UA.

I was told that they would cancel the earliest by departure time reservation (I had booked the Saturday reservation first).

Fortunately they could get me back on the flight I had been canceled from.