Nginx as reverse web proxy changes all to apache default page.

I'm very confused. I am using Nginx as a reverse web proxy in VM environment with 4 VM web servers. I have 4 conf files directing to each site. The last 2 mornings I have found that all 4 sites are defaulted to an apache start page. There is no apache on the Nginx machine so I assume it's showing the apache page from one of the 4 servers, I'm assuming it's the one listed as default in the conf files.

I have to reboot my router and the Nginx machine to get it to come back. I am using an IPCOP open source router.

Any ideas why this is happening?