Hm... I'm gonna take a guess at what happened:
Shou was flying over the lake, holding her Pagoda, when Rumia showed up and tried to eat Shou. Not noticing Rumia until she was close, Shou was rather surprised and dropped her Pagoda out of sheer surprise and fear. She then dove into the lake below to get the Pagoda back
<Sakana> Changed that to aqua for readability ^^