Groupon Software Development Engineer interview questions

A brief recruiter call followed by a Technical round. The I didn't do very well in the 1st tech round. I was asked to write an algorithm on a card game. I thought that was the end of it. But then I got a calling saying they would like to evaluate further and in the next tech round I was asked to write another algorithm and I did really great. They sent an email later saying they would not consider me, without giving any reason why.