Occurs due to over-exertion of magic. A stressed witch having to use their powers strenuously without rest in-between will soon suffer overload.
All witches will feel similar physical effects of exhaustion. Other symptoms may vary depending on what their speciality of magic is. For instance, a fire witch will have an uncontrollable fever, whilst an ice witch might have symptoms mimicking hypothermia. When the suffering witch hits their 'breaking point', so to speak, there will often be an explosion of their magic in the nearby area. This can obviously lead to there being danger to those nearby, especially with more destructive forms of magic.
Rest is often the easiest treatment if it's a mild case. In more extreme cases, where a witch remains overloaded and cannot perform any more magic, a ritual might need to be conducted to get them on the road to recovery.
It's very common amongst younger witches, excitable and just learning to get a hold of their magic, as well as those with more chaotic or unstable forms of magic specialities.