It's a passive progressive: a combination of a passive (e.g. "have been thrown away") with a progressive (e.g. "are/were being thrown away").
The combination of passive and progressive isn't so common, especially in more complex cases, and you might argue that such combinations are still somewhat "marginal". Indeed, some view that the emergence of such forms is an 'ongoing change' in English (cf Mair & Leech, "Current Changes in English Syntax" in The Handbook of English Linguistics, Blackwell, p. 320).
If you Google combinations such as "would not have been being", "had not been being", you will find other examples. Tellingly, the top results are from grammar guides and linguistic articles discussing the existence of these forms rather than "real-life" examples...!