This isn't something nefarious or suspicious, nor is it a bot. It is the Volume Purchase Program, either for educational institutions, or for large enterprises. The way these institutions distribute apps to staff internally is via some form of Mobile Device Management system. There are apps they have to distribute to staff/students whatever, and these are purchased in a way that shows up as a desktop purchase in the stats. This kinda makes sense as the person purchasing your app is most likely sat at a desktop computer putting the order through the system. Unlike a user purchasing and downloading your app, this is like a 'right to download' so many copies of your app, and not actual downloads. Looking through your analytics, you will see this to be the case.
Now, someone within an institution may have requested your app be made available over their system, maybe even just for a team/class of 10 or 20. However, for the person administering this system, if the app is free, it costs them the same amount in terms of their time to put an order in for 20,000 as it does for 20. Theoretically someone else could come along later and ask for another 30 copies. What are they to do? Waste time, putting in exact orders for the requested amount each time a request comes in, or block booking for the maximum they are ever likely to need and never having to repeat the action? You guessed it, they block order more than they will ever need.
This is annoying from a stats point of view, but there's really nothing you can do. X licenses have been requested for your app, but they are not obliged to use all of them. All you can really do is be aware that this is a 'thing', and maybe strip desktop purchases out of your stats (or at least strip out these huge one off purchases that are normally for a suspiciously round number). Very few normal end users make desktop purchases through the iTunes app these days. Certainly in my own reports, they normally just look like noise.
One other thing. If you have a paid app and put it free for a few days, I've found that some institutions seem to take these as opportunities to bulk purchase an app "in case" it might be useful for that organisation in the future. I've seen that the last few times I've done this. The first time it happened it nearly put me in a panic, along the lines of "if only I'd waited, maybe I'd have had 20,000 paid sales! 😮". It's important to remember that these are rarely "real" sales, quite often they're opportunistic, and wouldn't have happened if it hadn't have been for the free sale. In my book, running a free sale of a paid app for a few days is like the best form of publicity available - you get massive coverage for no outlay, just a few weeks of elevated support levels afterwards. And quite often there's a day or two of elevated sales after going back to paid which quite often covers the normal "lost" revenue. You just have to be aware that some purchasers for institutions are actively watching the "gone free" lists and taking advantage of them to get quality apps in bulk on the cheap.