This is a handy reference tool to help show what "normal" offense is in MLB and how it has changed over time. It is repackaged Fangraphs data - no earth-shattering analysis.
I don't know why or how things have changed from the beginning of the 2008 season until the end of the 2014 season in MLB. Analytics, defensive shifts, different approaches to pitching and hitting, changes to park configurations, recruitment of elite athletes into player roles, the decline (we think) of performance enhancing drugs -- there are lots of things that could cause some or all of this.