I believe the productivity of the yeast/sugar solution is affected with increased temperatures, the yeast is simply more active at higher temperatures it just produces more CO2. Being in the sun is probably warming it up.
That was what i thought at first too, so i put the co2 bottle in a cup with warm water to see what happen. It did produce a little faster bubble (1 bub every 5-6 sec), but nothing like when the sun hit it directl; i was getting like 4-5bps. Maybe something else is happening & not co2 production? any idea?