It means the person wants to be ready to eat any cake they happen to find. "Cake happens" is a way to say this humorously. Saying something "happens" means for it to become realized. Cake just doesn't "happen" ordinarily. The idea is that cake might just magically and luckily appear somewhere. When it does, this person will be prepared to eat it.

Ha ha - this means that one never knows where one might encounter a cake... for example, somebody on a train might have made a cake and offer some to the other passengers, even complete strangers. The author is saying that they love cake and therefore they carry a fork with them just in case they get an unexpected opportunity to eat some.

Hello Rafaela I must admit I have not heard this expression before but I would say that the meaning is that you should be prepared for any opportunity that comes your way. Best wishes Bob

@Neil I understood the meaning but I didn't get why is funny :s

The sentence you're asking about is really very funny and clever!