Dr. Martens' combat boots were adopted by counterculture movements, first in 1970s England and expanding to goth, punk, grunge and heavy metal scenes in Europe and the U.S. in the 1980s and 1990s.
It doesn't come as a surprise to see the "Complicated" singer donning combat boots on the red carpet as she's been known for her pop-punk style since rising to fame in the early aughts.