Recently, I played a game and after running the computer analysis, it seems like playing 5. Ba4 was a mistake, being better Bxc6.
[![enter image description here][1]][1]


  

However, I cannot find the reason why. By retreating the bishop, White does not lose a pawn after ...Nfxe4. So I cannot see why my move would be a mistake.


  [1]: https://i.sstatic.net/vWe2V.jpg