Dissolving NaOH in water is bad enough, but dissolving NaOH in acids (citric acid, lactic acid, vinegar, lemon juice) is much worse! You have to add the lye in many small steps, and wait in between until it has cooled down again. Had lemon juice in a cold water bath boiling.
The lye+water gives off heat two times: first during dissolving (saturated solution), and once again when the concentrated solution is diluted with the rest of the water (streaks rising from the dissolving lye beads). With lye masterbatching, you can defer the dissolution heating to another day, but the dilution heating is still there.
With acid, you have a third kind of heating: neutralisation (acid + lye → salt + water + heat). When all three heat sources come together, the result can be catastrophic, just as you have experienced.
@TheGecko is right that you should think about what has changed (also including stupid errors like weighed too much citric acid). But less so why this reaction now has occured (it is absolutely normal that lye behaves this way), but why it hasn't done so before (Old lye deactivated by air contact? Colder batch water?).
An obvious recommendation would be lye masterbatching, i. e. prepare a large amount of 50% NaOH in water solution beforehand, and store/use it at room temperature. That way, you save yourself from the dissolution heat. Or use (tri)sodium citrate instead of citric acid, which avoids the neutralisation heat.
ETA: Just saw that you replied. Don't use the microwave to dissolve the citric acid. It will do so all by itself (or you might prepare a 50% solution of it beforehand). If you're impatient, you could add a
tiny bit of NaOH into the citric acid (with heavy stirring), so that the neutralisation heat plays the role that would have had the microwave.
I don't have any ideas as to why that happened (unless your water was hot), but I'm sorry you were hurt.
