Cisco ASA Site-to-Site VPN Dropping
I think it might be the keepalive being disabled, if there is not traffic or traffic it routing another way it may be causing the tunnel to drop do to inactivity. Try dropping the tunnel (clear isakmp sa $PEERIP) on the destination then running debug on the source and see if it is trying to re-establish the connection. http://www.cisco.com/c/en/us/support/docs/security-vpn/ipsec-negotiation-ike-protocols/5409-ipsec-debug-00.html#crypto_isakmp