The issue involved someone, apparently in Russia (Russian language forum site), who discovered that the bytecoin miner (subsequently cloned to MRO) had apparently been de-optimized prior to public release and could be made several times faster with relatively simple changes. It was one of the MRO developers (NoodleDoodle) who promptly released an un-deoptmized miner (subsequently back ported to bytecoin), and later released additional optimizations.
You won't find a more fair launch of any coin no will you find team behind a coin with more integrity than the Monero team in my opinion (though as a minor disclaimer, I don't know all of them outside of our work on Monero -- the work on Monero has been 100% above board and community-focused).
What is the reason that the bytecoin miner had been de-optimized prior to public release?
Ask the bytecoin developers. Oh wait, they're hiding somewhere.
We can't prove this 100%, but several highly qualified programmers (including at least one not associated in any way with the Monero project) have looked at the code and found it difficult to accept any other explanation. Draw your own conclusions.