Yes, as Howard says, it is bad English. In fact there is a lot of bad English out there, so don't doubt your English instinct; if it sounds like bad English then it probably is.

I got why it is in that way. That makes sense to me. Thank you for your explanation.

I believe this phrase comes from the show "How I Met Your Mother" and is popular because of this. The grammar is incorrect in this phrase but it is still widely used because it was popularized on the show. I could not find any other origins for the phrase besides that show. As for "Challenge is accepted." This would be incorrect because there is no article at the front. You need to include a word such as [the], [a], [your], [my] or anything like that. So the correct phrase would be "The challenge is accepted." or "His challenge is accepted." You can still say "Challenge accepted" in casual speech because it is very popular. I hope this helps you. If you have any further questions, just ask. Best regards, Howard