AT&T interview questions

I recently completed a technical interview where I was given a HackerRank online assignment. The questions were quite challenging, at a medium-hard level on LeetCode, covering various algorithms and data structures, including complex graph problems

Four phases: pre-screening technical assessment, pre-screening behavioral, then a technical interview, ending with a behavioral interview with the associate director. I made it through the entire process very quickly and with good signs, only to get rejected at the end without any explanation. The process itself was fine, but a lack of explanation and an inability to defend myself against whatever reason they rejected me left a poor outlook on the whole thing.