If I had to guess, it could be for one of several reasons.

The first is that those stranges are just too expensive for most people for conventional trading, which narrows the market. That, and also that the weapons just aren't that popular. Minigun, especially for example, as heavy isn't that popular for one, and other weapons are preferable. I for one prefer the tomislav should I ever play heavy, considering it a superior weapon. Also why buy a strange minigun for 7 ref when you could buy a botkiller which is functionally identical to the stock, only difference being the name and the ugly arse robot head dangling below it, for 2 ref. Which is my best guess as to why the minigun is falling.

The grenade launcher is something that I rarely see in use in multiplayer, because why take the time to get good with a skillful weapon, when you could use the sticky launcher and go demospam? Basically, the market for these weapons is limited due to the current price, as well as being limited by demand, which makes me wonder why they are valued so highly, but if I had to guess it is that the weapons are from crates long before the time of key inflation, and the price does not change is the price of the currency changes. So the drop is arguably long overdue should this be the case.

Another reason is that traders have lost faith in these stranges. The Grenade launcher has only been unboxable in the 19s, so I for one think it is long overdue that valve releases another crate with these in, which would of course plummet the value of the GL, or any other strange should they be unboxable in a new, readily available crate. A friend and I actually sat down one night and went through all the possible new stranges, and there aren't all too many. Mark my words, the GL will return in the immediate future as an unbox. (Sidenote, but also looking forward to the heatmaker, for srs tryhard reasons, the short circuit, for da luls, and of course I'm still holding on to the dream of my beloved strange Postal Pummeler) As a result, I think it one of the reasons for the GL dropping is that people share the same mindset, similar to the way 'weeny effects are dropping a bit, as people want to get rid of them whilst they still have the bonus of being limited numbers, in preparation of the probable unboxes this halloween.

Traders could also have lost faith in them for the reason that they are dropping, so people want to get rid of them at "quicksale" prices, but no one wants them enough, so these prices get incorporated in to the price range, and the cycle repeats.

Salvages come under the first reason mainly, but a bit of people can wait to buy them, as everyone thinks they'll only go down. Most of them are niche weapons, pretty much. So why pay out the arse for something which is pretty dumb and not normally used? Notable examples are the Strange Pan, caber, mantreads, holiday punch and Splendid Screen. These are luxury items, only really bought by those with too much to know what to do with, so supply tends to outweigh demand.

TL;DR? Nah, read the wall of text :3