Xiphactinus is sometimes referred to as a bulldog looking fish, which makes sense when we look at the jaws of the fish and how they are turned upward at the same time as they are jutting out into space ahead of and below the eyes. The length of the fish was imposing even without the teeth; the longest estimates reach approximately 20 feet while many known specimens reach at least 16 feet. That much fish was probably difficult to turn, however, with the body build that the big bony fish had and the lack of maneuverability probably allowed more than one smaller fish to elude the grasping and deadly maw of this large fish. The only dangers left in the water to a fish this big would have been sharks and mosasaurs, and size would be the only protection against mosasaurs while agility would be the best defense against sharks of near equal size; Xiphactinus apparently was not adapted to outperform either exceptionally well however.
I think, given the size and lack of agility apparent, though we said nothing of speed and I am willing to lay down a small bet that Xiphactinus had a barracuda like surge of power at least inherent in its physiology, that a school such as this would have been rare or non-existent with the exception of breeding season movements to spawning grounds. Considering we know so little about the early stages of the life cycles of these fish anything is possible, including mass movements of the fish during the spawning season to a seasonal spawning ground. Looking at their size and the lack, so far, of evidence of any freshwater habitation, we can conclude for the time being that these were not salmon like in their need to return to a river system to breed, but where then, would a school of Xiphactinus go to breed, if they did ever "school"? Either way, these Xiphactinus are on some kind of mission and are very intent on making it to wherever it is they are going.
Probably one of the most famous, if not the most famous of Xiphactinus fossils, is the fish within a fish. Discovered by George Sternberg. The fish in its gullet is a Gillicus, a 6 foot fish that, the main theory holds, writhed in its death throes and ended up effectively suffocating the larger Xiphactinus. The only way for the preservation to be as fantastic as it is, honestly, is very quick burial within the sediments at the bottom of the ocean. Had the burial been slower or the corpse not drifted to the bottom of the ocean quickly then it would have certainly been scavenged, torn apart, and not as fantastic a fossil as it is now, obviously.