This may or may not be the case.
Short of having conclusive evidence beyond:
"I dropped in SERP"
"My PageRank is down"
"My backlinks is down"
you can not conclude "a major algo-shift".
Careful observation produces evidence, not mass fluxuations.
Google has added 6 hundred plus million pages to its index, this tends to suggest most pages previously indexed are now competing with more, not the same amount and certainly not less.
Be careful what you submit as "fact". :)
In theory a algo changes should produce a 50 - 50 split of up's and down's rather than all on one side or the other.
Unless of course we are all doing the same thing, right or the same thing wrong.