Found some info!
This study:
https://ir.library.oregonstate.edu/xmlui/handle/1957/10869 mentioned that it's a pioneer plant, doing well in disturbed, "high productivity" soils, sprouting best with lots of nitrogen, but being less likely to become perenial when there's lots of nitrogen.
This document on Permaculture News:
http://www.permaculturenews.org/resources_files/pdc_info/Plants_as_Soil_Indicators.pdf says foxglove indicates low lime (so acidic and low calcium, right?)