Are you running VPN monitor options? This will bring down VPN interfaces when they fail the monitor test.
If not, I think you best bet for a root cause here will be setting up trace options and getting these logs. This is the route based vpn trace logs setup. These should give us the specific reason the SRX is taking down the interface.
https://kb.juniper.net/InfoCenter/index?page=content&id=kb21781#IpsecRouteBased