Congratulations! You had something with nitrates and some form of oxidiser. Been in a garden or around fertilizer recently and then used hand cream or a cheap glycerin soap by any chance? Or, more likely, was the TSA "officer" wearing gloves they'd run through their hair that contained a styling product that then touched cheap nylon baggage?

Clean gloves are important. Re-using strips is less important as the heat from each test basically cleans it each time, although proper protocol is to test/clean the strip first if you're going to re-use it. (The tests work by heating the strips very rapidly to very high temps to vapourise any nitrates or phosphates for detection and measurement.)