Big Crate of Salvage was randomly rewarded for successfully completing the higher-level garrison missions while owning a Salvage Yard (Level 3). In Patch 6.2.0, 
Raw Scrap was added to replace the random gray vendor trash.
While it is no longer obtainable, the data is left here for historical purposes.
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Posted Oct 18, 2014 (Patch 6.0.2)Alongside the work orders from the Dwarven Bunker / War Mill, the crates from the Salvage Yard are the best source for follower item enhancements. These follower item enhancements are required to get access to (and complete) Garrison Missions that reward Normal, Heroic, and Mythic raid gear from Highmaul!
The types of salvage crates are:
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Posted Jul 6, 2015 (Patch 6.2)Patch 6.2 brought a couple notable changes to the Salvage Yard and crates of salvage.
Ion "Watcher" Hozzikostas (a.k.a. Ono Fuzzikoalas) stated that the team did not feel comfortable with alts being used for professions and additional Salvage Yards (related to treasure hunter trait and gold-rewarding garrison missions, too). This sentiment was echoed by a Community Manager who explicitly said that choosing to not have a Salvage Yard was a case of "you're doing it wrong."
Raw Scrap. :(
Moonglow Vest.
For a comparison of Salvage Yard Level 3 post-6.1 and post-6.2: http://www.youtube.com/watch?v=hr4enGK8Tcc ("6.2 Opening 900 Big Crates of Salvage! Statistics (After nerf) & Comparison to 6.1 Unboxing!")
Notes: It is indeed possible to open a big crate and loot a single
It is also possible to loot profession-crafted items, e.g.:
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Posted Jul 13, 2016 (Patch 7.0.3)Replaced in Patch 7.0.3 with
Large Crate of Salvage.
Rest in peace, Big Salvage. You were beloved by collectors and will be dearly missed.