Chess Opening Theory/1. e4/1...e5/2. Nf3/2...Nc6/3. Bb5/3...f5/4. Nc3/4...fxe4/5. Nxe4/5...Nf6/6. Nxf6/6...Qxf6/7. Qe2/7...Be7/8. Bxc6

Here's an example of how chess opening theory moves with the times. Why 8...bxc6 and not 8...dxc6? The intelligent answer is something to do with capturing towards the centre and pawn islands. The honest answer is that Black players tried 8...dxc6 and laid an egg, so they tried something different.
 * 8...dxc6 was by far the more common move until the mid 2000s. It seems logical to open a diagonal for the bishop that it can actually use, right? The same exchange in the Exchange Ruy Lopez is always met with dxc6.
 * But then along came Teimour Radjabov and revived the whole 5...Nf6 line based on the innovation 8...bxc6, and now every top player who tries this line - Zvjagintsev, Timmerman, Fressinet - goes 8...bxc6.