UV lighting is a form of radiation (very low) but still strong enough to kill microbes. Thus, it could make plausible sense for the frogs to naturally be drawn to the light, plus the increase in fertility could correlate with healthier adults. I should add that UV can also damage parasites and fungi that could be inhabiting an amphibians epidermal layer. Many amphibians have beneficial peptide compounds that lysis foreign invaders on their ectoderm. The brief 20 minute period of UV exposure then would be similar to what we humans do when we radiate food. Which is hitting the food with charged elements that acts as an antimirobial, fungicide and antiparasite. Sorry for the nerdism haha, Pre-Vet student with a minor in Chemistry and Physiology, can't help it :-).