One answer might be that churches used to have several altars – the high altar and altars around the apse as well as on either side of the main altar and sometimes in bays along the aisles. These bays are often shrines to particular saints. In the days of yore, when churches had more priests than daily Masses (before the vogue for concelebration), Priests could say their required daily Mass at one of these side altars. Nearly every church had one dedicated to Our Lady, and that would be “Mary’s altar.”
Another possibility is that when people construct a little devotional shrine with a statue or icon, candles, and perhaps flowers, they adopt the term “altar,” even though it isn’t an altar at all.