Why the sudden rise in usage of "吓死宝宝了"?
I think it's because of the Internet, which made this sentence popular. There was once a boy in Sina Weibo (China's version of Twitter) who sent a threat to a girl. The girl on Sina Weibo replied to express her dissatisfaction. The message was "吓死宝宝了". 

It soon got around on Weibo. Many people were talking about this. So now many people use this sentence to express their discontent by force or threat.