Just a guess, but it probably had to do with the high premium (still +2.3% AFTER the 5% swoon you describe).  Also, distribution yield is a relatively non-competitive 4.95% - again after the 5% price drop, and per Blackrock's monthly update on muni CEFS, although it is currently earning its distribution, the average negative UNII for the past few months has been about 7 cents.

Or it could be that somebody just foolishly placed a market order to sell a big slug.

Regards, Dick