The most common causes of nosebleeds that occur suddenly in adults is due to dry air or high blood pressure or just simply picking your nose.
Our noses contain tiny blood vessels which are easily damaged or ruptured and then when the nosebleeds are damaged or ruptured they can cause nose bleeds.
When the air is dry your blood vessels can dry out and crack and then allow your nose to bleed.
High blood pressure can also cause your blood vessels in your nose to rupture which then also leads to nosebleeds.
If you're just getting nose bleeds suddenly once in awhile and they stop soon then it's usually nothing serious but if you constantly get nosebleeds you might have an infection in your nose or have some other serious health issue and should see a doctor about it.
Having the cold virus can also lead to nosebleeds but usually it's due to dry air or damage to the noses insides that break the blood vessels.
In some rare cases it might even be a sign of cancer when you have really severe nosebleeds but usually it's just nothing serious.