A large part of that is driven by ETH and XRP though (and to a lesser extent XEM and STR), where the majority of available supply is known or suspected to be controlled by a small number of folks. The rise in these coins seems more like a coordinated pump than money actually fleeing BTC imo. To be sure BTC dominance is eroding as more coins find users and speculators, but I suspect this recent drop in the dominance index will correct strongly in fairly short order.

If you look at the all time BTC dominance over time here http://coinmarketcap.com/charts/ you can see that each time there has been a sharp drop it's followed by some correction back up over the following weeks/months. The drop this time was especially steep, but I think the correction will be as well (especially if BTC itself starts to push above and beyond $2k USD).