One of the causes may be that the unit cannot get rid of its heat. There may be a blockage in the hose due to too many bends and/or sharp bends, for example. A hose that is too long (by lengthening the original hose) can also be the cause.
Another cause is that the hot air cannot get out because the hose is not hanging outward or is constricted.

Note: Always use only the original, supplied hose and make sure it is placed directly outside. Using another hose or extending the original hose may cause a malfunction and this will also void the warranty.