"Damn, invisible ninjas are chopping onions!" is a meme that describes people who hate to cry but are forced to do so by something romantic or sad.
Also, some people even said just ninjas. Not invisible! They also said "Cutting", not chopping. But chopping is kinda better to some folks...
And this is one of the versions of this ridiculous baby meme:
" HELP! THOSE INVISIBLE NIJAS ARE CUTTING ONIONS ONCE AGAIN!!!"