Drumming is pretty much analogous to singing, so they are doing it mostly to proclaim ownership of a territory. It's most common earlier in spring, but they drum all throughout nesting season and can do it occasionally at other times of the year. My guess is that this Downy just found the perfect drumming spot and she likes the sound it makes, the louder the better! It could be that she was drumming in a different part of the territory and/or on a less resonant surface before, so the sound wasn't as loud before in your location. Or, this pair lost their territory to another pair, and have had to move to this new location. The frequency of her drumming should subside in time, but once they have found a good place to drum, they usually make a habit out of it.
Lisa Millbank