Amazon Software Developer interview questions

I had two phone interviews. First phone interview went okay. The interviewer asked me to write a code which I did correctly but I kept getting confused with complexity a tiny bit. His question was to merge two sorted arrays in another sorted list. I didn't sound very confident in the first interview but overall experience was ok. I was then called for second phone interview and it went extremely well. I answered all the questions correctly. The coding question was very nice but I came up with a very good solution. The interviewer was complimenting me a lot. However, I got a rejection email after two days. Hence I don't know went wrong :( I don't know how these companies hire. I would have understood if they rejected me the first time but why after second interview especially when it went so well.