"Bag", as a verb, is used amongst mountain climbers to describe a successful ascent of a peak. The derivation is from the idea that the climber has put the mountain in a metaphorical "bag".

The term to be has somewhat of an unsavory connotation, perhaps because it can also be used as a term of sexual conquest. While I enjoy the outdoors, the term "bag" makes it sound as if nature was something to be conquered, rather than experienced; which is an unfortunately common attitude amongst some mountain climbers.