In some cases yes, major changes to the websites structure, can shake the rankings very hard. For instance, assume dozens of your internal content pages have their own incoming links and then suddenly you change their URLs, move from HTML to PHP. Naturally the links will not seem that relevant in such cases. Or when you quickly alter titles and descriptions of dozens of pages on your website, the ones already cashed and indexed by Google and use completely new stuff, you can expect a drop in rankings in such cases.
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