I'm 5ft 7 and 127pounds but occasionally I have a double chin on candid photographs, why is this so? And how can I reduce this? Thank you!

Bad photographer? or bad photography? Nothing to worry, you are in great shape.
Variability. "Occasional" double chin likely related to your head position (chin down) when photos taken. Or if a slender person like you has a small chin, sometimes skin below chin less "taut". If you really want to "reduce" this, the only option would be surgical. See a facial plastic surgeon.