It happens all the time. The car was originally in Park and charging the battery with engine RPM above 500, which is roughly 1,000 alternator RPM. When a load was put on the engine by shifting out of Park into Reverse, a sudden load was put on the engine, slowing it to below 500 RPM. Below 1,000 alternator RPM, most alternators won't "excite" and produce current, and the system is then on battery power momentarily. A fully charged battery is about 12.7 volts, while a normally operating and well-regulated alternator will run in the range of 13.5~14.5 volts. It does sound as if your IAC valve is getting sluggish and needs cleaning/replacement, to respond to engine loading faster than 5-10 seconds.
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