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[ReneZ]

But I can't find any probable reason to create a new script in any scenario, so whatever the truth it is likely to be improbable. 

In the Middle Ages people were taught the works of Isidore of Sevilla, and Hrabanus Maurus, and both taught, in slightly different versions, that:
- Hebrew writing was invented by Enoch - Cham - Moses
- Syrian (Syriac) writing was invented by Abraham
- Greek writing was invented by Cadmos (with later additions)
- Roman (Latin) writing was invented by the Nymph Carmentis.

Obviously, nowadays we do not learn such fables in school, but that was what people were taught in the middle ages. It is probably what prompted someone called Aethicus to invent his own alphabet around the second half of the eight century. This alphabet was listed in a famous work by the traveller Mandeville, where he called it the C(h)aldaean alphabet.

One tentative motivation therefore presents itself: to be known as the inventor of a new alphabet, and get your name on that list.