BGP failover and redundancy with two circuits

Hey guys, couple question here any help is appreciated!

Currently I have two MPLS circuits both running BGP. I am testing failover and redundancy to make sure that if one circuit goes down traffic continues to flow. What I noticed is when I shut down the MPLS interface on one of the circuits about half of my sites lose connectivity to us and it takes 90 seconds to come back up. If I shut down the other circuit the same exact thing happens to the other sites.

My questions are:

1. We have two routers running MPLS with BGP advertising the same subnets. why would a site select one router over the other for bgp?

2. Default Hold time is 180 and keepalive 60. Currently its set to 90 and 30 seconds (which explains the 90 seconds for the sites to come back up). Can this time be lowered any further without causing issues?   

3. Last question is there a better way to set this up to load balance traffic and minimize downtime? Would looking at setting up BFD help?

Thanks for the help!!