Arriving home after a 2 day absence, when I opened the refrigerator, the interior was 98 degrees F (food ruined). The side-by-side freezer was normal temperature, but it was evident it had thawed for a period and then refroze (e.g. ice cubes were melted together). The Control panel lights were OFF. When I tried to reset the temperature, the control panel lights came on showing the current temperature, and then started cooling. After a couple of hours, the refrigerator temperature was back to normal and still is 1 day later.

What caused this interior heating, and how do I ensure it does not happen again?

You may have had a power outage, did you notice if the clocks were off the correct time? If the power failed just prior to a auto defrost cycle you can get a build up of frost that blocks air flow between the two compartments. Once the freezer air can't reach the refrigerator compartment the temp will rise. Never saw one reach 98 before but there are a lot of things I've never seen yet.