Infection, medications, rhinitis, alcohol abuse, hypertension, tumors, dry climate. Ask your doc to find your cause or refer you to an ear, nose amp; throat to get to the cause. A newer cause can be a problem called MRSA (an infection). Often located in the nose causing bleeds and requiring an antibiotic. Ask.com people surely are helping with suggestions.
I know this is an "old" question but I wanted to add one more to the mix... I have severe allergies, and have found that some over-the-counter medicines AND prescriptions for long-term control of said allergic reactions (Allegra, Zyrtec, etc) caused me to get daily nose-bleeds. I finally found one that worked for me, but it took me a while. Hope this helps, and I hope you figure out the cause soon. <3