What causes a change of mission type during a play-through?

I was doing a mobile defense mission on Sedna, and after the second defense point, Lotus said something to the effect of "Change of plans. Ignore the original mission and eliminate all enemies," and the mission changed to an exterminate instead. What caused this change?

This appears to be a type of Second Primary Objective. These were added a while ago (listed as "mid mission surprise changeups") and were designed to "make things more interesting."

After complaints, the frequency of this event was drastically reduced, which lends itself to the idea that it is triggered by something, but it appears to be completely random, and usually decided at the start of the mission.