How do unisex names come about in English?

TL;DR

How do unisex names develop amidst, or out of, gendered ones in English (and other languages)?

Detail

In English, many (most?) names have a gender assignment of male or female. However, some are considered unisex (e.g., ‘Sam’, ‘Alex’, less contemporary examples such as ‘Evelyn’). Assuming that English generally moves from more-gendered to less-gendered rather than the other way around, I am wondering what the mechanism by which certain names are deemed unisex (or come to lose their gender exclusivity) whilst others do not.

I’ve come up with a few theories, but none of them entirely convince.

• Someone notable bucks the trend and inspires others, e.g. the increasing popularity of ‘Rory’ for girls following Gilmore Girls. However I could only really find one example of this.

• There’s some linguistic/phonemic/morphlogical reason for the male-female allocation, and unisex names bridge this in a way that makes them intuitively unisex to native speakers. However, the fact that natively-English names (in true English fashion) don’t seem to follow any consistent rules (e.g., ending in ‘-a’ or ‘-o’ to denote gender as in ‘Roberto’/‘Roberta’), and that certain names have different genders in other languages (e.g., ‘Andrea’ being a male name in Italian), seem to suggest that this isn’t the case.

• Perhaps, following on from the ‘Andrea’ example, encountering other cultures with differently-gendered names (or similar names, or false friends) led to a reduction in gender-association with those names in English. However, that evidently hasn’t worked in the case of ‘Andrea’, and foreign names from further afield than Continental Europe seem to bear too little resemblance to English names.

• Many unisex names are diminutive versions of gendered names (e.g., ‘Sam’ for ‘Samuel’/‘Samantha’), and as a result they must have come into existence referring to both genders initially (i.e., they are additively, rather than subtractively, unisex). Being colloquial would presumably also allow such names to develop and spread more rapidly than formal, traditional ones, in which case progressive societal/cultural influence may be more pronounced on them. This seems the most convincing explanation I’ve come up with, although doesn’t account for many non-nickname unisex names and also doesn’t seem to consistently apply to nicknames (e.g., ‘Andy’ is short for ‘Andrew’, but never ‘Andrea’ in my experience).