Reasons to Double Cap
(1) secure their goggles on their head by sandwiching the goggle straps between the two pairs of caps,
(2) improve heat retention through their head especially if a bubble cap or neoprene cap is used,
(3) place a required race cap on their head over their favorite swim cap, and/or
(d) ensure that at least one swim cap stays on their head during the swim.
Double capping can include silicon over latex, bubble cap over silicon, neoprene over cloth, or any other possible combination of the different types of swim caps.
The swimmer was double capping when she put a neoprene cap over her regular latex cap because of the cold water.