SUDDENLY is the tag for when the plot development of an illustration or manga unexpectedly goes through the roof and beyond.
While there is no specific meme term for SUDDENLY, in Japanese, it is called "超展開", which literally means "super development", or much more suitable meaning, "sudden plot development".
Related Tags
Ends With a Flash(衝撃のラスト), WTF Happened(何があった), WTF(どうしてこうなった)