Learn English – How many words/phrases were coined by Milton

Actually Etymonline suggests that the first use dates back to 1917 and is referred to trucks loads while its military application was later, around 1936.

Payload (n.)

also pay-load, 1917, from pay + load (n.). Originally the part of a truck's (later an aircraft's) load from which revenue is derived (passengers, cargo, mail); figurative sense of "bombs, etc. carried by a plane or missile" is from 1936.

Ngram shows that military payload has gained currency by the mid 40's and that suggests that the probable origin is from a truck's load.

The increase in use of the term in the 60's is hard to detect since unluckily Ngram does not help from a semantic perspective. Aircraft payload, passenger payload or industrial payload don't show significant increase in that period.

• Further research ( with the help of StoneyB) shows that actually the term is much older. Paying load" vs "dead load" is found in discussion of railroad freight as far back as 1849 and becomes "pay load" by 1903 where it refers to naval vessel's 'pay load' as its armor and armament.

It is also interesting to note the current meanings of payload:

• payload(Noun) That part of a cargo that produces revenue

• payload(Noun) The total weight of passengers, crew, equipment and cargo carried by an aircraft or spacecraft

• payload(Noun) That part of a rocket, missile, propelled stinger or torpedo that is not concerned with propulsion or guidance, such as a warhead or satellite.

• payload(Noun) The functional part of a computer virus rather than the part that spreads it

• payload(Noun) The actual data in a data stream.

First my "coincidence" answer

Animal metaphors for people are common. Cats are a familiar species and likely to be the source of many and varied metaphors (see also farm animals and dogs, but note that cats are more independent than either and thus more suitable as a metaphor for a person with agency). It's quite likely that (in a strongly gendered society) there will be a bias towards references to one gender; this bias will tend to be reinforced by usage).

Now the (not mutually exclusive) answer based on sexism

Cats, in contrast to dogs, are seen as cunning, employing trickery rather than good honest brute force, as well as sleeping a lot. Their sounds are also either high-pitched or gently soothing (though purring can be a sign of fear), while dogs make more aggressive noises. Cats are also prone to appearing as if by magic and to staring at things people can't see (a little fly in front of their nose but the other side of the room from their human) as if watching fairies. These all fit with human gender stereotypes, where clever women were seen as manipulative and/or witches, and women who worked hard behind closed doors seen as lazy daydreamers.

Thus many of the traits associated with women are similar to those associated with cats. While foxes display some of the same traits, they're less familiar and when confronted with an angry human male will flee while a (fully domesticated) cat will purr and stick around.

This is of course speculative; to back this up with usage would be tricky as we're going back a long time in a case where oral patterns matter.

Some history

Etymonline dates "catty" to 1886 without gender as "devious and spiteful" (ngrams shows plenty of earlier use).

Oxforddictionaries.com has "she-cat" as (¹a female cat or) ²a spiteful woman, with the origin given as "Late 16th century; earliest use found in Thomas Lupton (fl. 1572–1584), political and religious controversialist." It's not completely clear that this first use applies to the second meaning.

Shakespeare can normally be called upon for colourful descriptions of both men and women, but despite his "using them as metaphorical tools to express the opinions of women and to describe women’s nature" (basically a blog) the uses I can find aren't general about women; some are about men: opensourceshakespeare.org search results for "cat".

A few centuries later, Dickens appeared to like his cats (in fact he had his own cat stuffed after it died) but his uses seem to have been alomst all literal; the few figurative uses I've are as likely to apply to men as women.

This has been studied

If you have journal access, the cat as a metaphor has been studied:

• comparing English and Malay with no mention of gender (but the bibliography looks helpful).
• A study of English and Spanish use of anmial metaphors for women. The section starting

  3.1. Women as pets

  Within the animal world, pets enjoy a privileged position. In fact, pets are not conceived of as beasts of burden nor are they killed for their meat or skin. On the contrary, pets share the same roof as their owners and their main function is to keep people company. This benevolent attitude towards pets finds its way into the English language, for the very word pet is used as a term of endearment

is likely to be of particular interest (some woman-cat metaphors have Spanish counterparts).