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I agree with both responses, I would like to add my experience to the mix, my site decreased in Domain Authority and Page Authority due to the loss of a few sites that were no longer linking in a few months ago, my site is relatively new and the Domain Authority went from 20 to 18, it has since increased to 19 after a few new sites linked in so that could well be the reason for your DA drop, I am not aware of any recent Google Updates but I do know that my Moz DA was updated a few days ago so that may coincide with your recent drop, I am not sure how often Google updates the link data in GWT so it may vary from what you see in OSE and Majestic SEO, as Chris says, the historical link option for Majestic SEO is very handy and the data seems to be updated quite frequently also. Hope this helps