There are many reasons that a person may be experiencing this kind of anxiety. Usually there is something that triggered the anxiety, the person just does not know what it is. For example, they may have had something happen a week or two ago and they are just now feeling it compared with other forms of anxiety where the person is living through the experience right now and can pinpoint it.
Just like with other forms of anxiety, unexplained anxiety is going to have many different causes. These can include fatigue, withdrawal from drugs, hangover, increase in the amount of responsibilities the person must deal with, illness, and being done with a relationship. These are a few examples but basically anything that causes stress to the person, such as a change in their lifestyle, can cause them to have this kind of anxiety. The issues can become more prevalent if the cause occurred quite abruptly. These things could have happened a bit back in the past, but they are not finding the person and causing them to have some anxiety.
The causes of unexplained anxiety are a bit more difficult to pinpoint simply because they can happen at varying intervals before the panic attack occurred. They may have happened just a few days ago while others could have happened further back. In addition, since there is some delay between the event and when the panic attack occurs, it is possible for more than one event to cause it.
Luckily, even though the causes are hard to pinpoint, this type of anxiety is one of the easiest to treat and work through. The patient will be able to get through the anxiety with the use of interoceptive desensitization, panic control therapy, and some forms of relaxation. Being able to sort through your feelings and to take some deep breaths can do wonders on this type of anxiety.