Often this is a natural progression of events. But then it becomes worse because someone is to blame.
Because one partner in a relationship switches off does not mean that the lack of sex is their fault. That analysis would be like accusing symptoms of an illness as being the illness itself.
Symptoms and causes are different. Sometimes there is a connection between them and sometimes there is no connection. Blame and blaming simply do not get anyone anywhere in sex therapy.
Sometimes a normal relationship will become non-sexual for a range of reasons. These can be physical, psychological or otherwise.
For example one of the partners could devlop an embarrassing rash or fungal infection and avoid sex so as to avoid talking about it. Tragically, these minor ailments can be cured in a few days with the use of a readily available ointment but this could destroy a relationship.
The complexity of reasons for sexual breakdown are legend. The most innocent of events can cause sex to be turned off.