Origin and legal status
Crush pornography probably originated in Japan, and was popularised in the 1990s by a single website that retailed DVDs. At the time, the material was technically legal in many countries, because the definitions of "Animal" in cruelty laws did not extend to invertebrates (reference needed).
Illegal material featuring animals also existed. This was difficult to prosecute because the actresses were anonymous. In 1999, the US Congress introduced a new law to prohibit the *portrayal* of intentional cruelty to animals (including bugs).
This had First Amendment implications .
The news media frequently suggested links between crush pornography and foot and high heel fetishes. The distinctions from BDSM are that animals cannot consent and that Crush Pornography is adding these acceptable fetishes to an undercurrent of passive necrophilia. (e.g. see "Wormee's" comment in the Hustler article).
Composition and Viewpoint
Videos that were available in the late 1990s typically included no introduction or back story, opening with the creatures already crawling around on a tiled floor or similar.
The women's faces were normally kept out of shot. This may have been to reduce the risk of prosecution, but the material was potentially legal in many countries at the time.
An alternative explanation would be that this was done to make the viewer identify with the small creatures.
Eraserhead (David Lynch, 1977) includes a scene where an imaginary girl living in Henry's radiator crushes (claymation) worms on a stage.