You wouldn't have to physically drop the backpack in the game, just that you wouldn't have access to it as if you had dropped it. You can use any reason you want, like - Being surrounded by Vlish and Clawbugs you just don't have time to dig past the shaper equipment, spare weapons, and extra armor to find that speed pod you put in there earlier.
"Don't attack me yet, I found it, now just give me time to move these five iron bars out of the way so I can... AAaagh!" oops your dead.
ie. While in combat mode trying to open your backpack would do absolutely nothing until combat has been ended.