All migraine is chronic, but there is a specific diagnosis of Chronic Migraine which is more than 15 headache days a month, 8 of which are migraine days. You may hear the phrase 'transformed' migraine, which refers to when someone with an episodic presentation of migraine attacks - less than 15 headache days a month - gets worse and has more frequent attacks, becoming Chronic Migraine. This happens to about 3% of the migraine population every year.
It is believed that people with migraine spiral into more frequent headaches and migraine attacks because of a process called Central Sensitisation. Frequent pain changes how the brain works, resulting in more pain being experiences with less provoking stimuli. The brain becomes more sensitive to pain and therefore you feel more severe pain and more often. The more frequent the attacks, the more this central pain sensitisation becomes persistent and difficult to treat.
The other factor is called medication overuse headache. This is a complication of migraine where frequent use of medications creates more headache. Again, the more medication you take, the more headache you get.