I have heard of clown fish doing this dive technique at very very baby early stages of their lives when they are going to die just one of those things unfortunately, my guess would be the frogspawn sting was too much and your clown hadn't built the immunity slime coat to the toxins.
175Ltr Aqua Marine 900, Marisys 240 Wet/Dry,
24W UV, 17.5KG Fiji Live Rock, Temp - 25C,
SG - 1.023, Amm - Nil, NitrI - Nil, NitrA - Nil,
Ph - 8.4, Alk - 4.3 Meq, Cal - 450, Mag - 1340