The big tech cos all do announcement extravaganzas at their annual conference day. MS just had theirs, which is why so many of their stories have appeared here recently.
I wish they wouldn't, because every time, people complain about MS/Apple/Amazon/Google/whoever using evil powers to stack HN. It polarizes the community: the fans upvote and comment while the anti-fans flag and grouse. But it's the way these companies do things, and some of the announcements are major ones, which it's not in HN's interest to suppress.
We raise the moderation bar when there is a flurry of submissions on any theme, so you're actually seeing fewer Apple/Google/Amazon/MS/whoever stories than you would if they doled them out over time—but the impression is just the opposite because they're so concentrated.
Edit: if you look at the following you'll see examples of how this comes up every time and how it rotates through the different BigCos:
https://hn.algolia.com/?sort=byDate&prefix&page=0&dateRange=...
https://hn.algolia.com/?sort=byPopularity&prefix&page=0&date...
