Problems when one internet connections goes down

Hi,
A few month’s ago i set up 2 internet connections (ISP-1 and ISP-2) on my Sonicwall 2040 Pro.
I use ISP-1 for my servers and ISP-2 for my users.
Everything work ok, till a few days ago I noticed that when ISP-1 goes down, my users don’t have internet connection till ISP-1 come’s up again.

What could be the problem.?
I think i could be a DNS problem, but don’t where exacly to look.

We have a windows 2003 server, which is our DNS and DHCP server.
User’s and servers are on the same subnets.