His blood pressure is overpowering the flow of fliud through the IV, which causes back flow of blood into the line. If the fluid is not consistently pumping, then this is usually the cause of back flow of blood. Make sure the fluids are running all the time, and also your vet should be having the technicians checking the line every 1-2 hrs to ensure it is properly placed and the line flushed when blood flows backwards like this. Also when the fluids are not running, the line should be clamped off at the level of the catheter to ensure no back flow of blood. There isn't any serious danger here if some blood gets into the line, but it is just preferred to keep everything flowing towards the patient and not away. Plus it is more sanitary to keep blood out of the line. Best wisehs with your pet and I hope he gets over whatever illness it is that he has. Take care.