A geologist working in the field needs a container for rocks specimens:
The collecting-bag may be in the form of a pouch to carry at the side, a knapsack, or a rucksack. If one is working afoot the former, when loaded with specimens, soon tires one's shoulder. A knapsack may be so arranged with straps and buckles that it may be readily converted into a bag to carry at the side - if one objects walking through town with a bag on his back.1
The United States Geological Survey was established on March 3, 1879, some twenty years before the above statement, so I assume that the geologist was a known and not unusual profession.
Maybe carrying a knapsack would have been associated to illegal behaviors like the ones of burglar or tramp?
1JOHANNSEN, Albert. Manual of petrographic methods. McGraw-Hill Book Company, Incorporated, 1918. Available at archives.org