Might be that you were putting too much load on one of the PSU's rails. The GTX 285's are very power hungry cards. Just to make sure, you did connect the 6 and 8 pin power connectors right?
Since your PSU is modular, you can look at the manual or on the back of the PSU as to what connectors connect with what rail and re-work the powering scheme to ballance the load on the rails.
Since your PSU is modular, you can look at the manual or on the back of the PSU as to what connectors connect with what rail and re-work the powering scheme to ballance the load on the rails.