What is the most likely reason for having an abnormally high blood urea level?

Co-tango usually: The BUN is usually 10 times the creatinine. When the creatinine goes up, the BUN goes up similarly to roughly keep this proportion, in most kidney disease states. The notable exception is when the creatinine goes up say by 20% to 1.2, but the BUN goes up *5 to 50. This strongly suggests a pre-renal state, commonly dehydration. Mild elevations of BUN with normal creatinines aren't of concern.