Shares of unmanned aerial vehicles (UAV) maker AeroVironment (NASDAQ:AVAV) stock are falling out of the sky today, tumbling 8.7% as of 10:35 a.m. EST.You can blame investment bank William Blair for that.This morning, analysts at William Blair downgraded AeroVironment stock from “outperform” to “market perform” (i.e., from buy to neutral), as noted in a write-up on StreetInsider.com. AeroVironment, which specializes in a subset of the military UAV market that’s designated “small unmanned aerial systems” (SUAS), actually builds many different kinds of drones for the military — but its best-known and most popular drones are the Raven small surveillance drone and the Switchblade, a small UAV that carries a warhead and attacks targets kamikaze-style.And it’s Switchblade that has William Blair especially worried today.As the bank explains in its write-up, the Pentagon’s 2021 defense budget requests funding in the amount of $5.45 million to “qualify” a second “LMAMS system” for use by the military. Currently, Switchblade is the Pentagon’s LMAMS of choice — the term refers to a “lethal miniature aerial missile system.” It’s also been the source of some pretty sizable defense contract wins for AeroVironment. Thus, the prospect that the Pentagon may be seeking a second (or even a third) supplier of such kamikaze drones (Raytheon is known to be developing one) poses a direct threat to AeroVironment’s monopoly in this UAV segment.William Blair also mentions in its note that AeroVironment’s Raven surveillance drone appears to be losing market share. Big as the Raven business also is for AeroVironment, however, it appears William Blair’s worries focus primarily on Switchblade. As the analyst warns:”We followed up and interviewed over the phone a member of the Switchblade contracting office from Army Contracting Command Redstone,” says the write-up, and what the official told William Blair is that “an RFP for the second supplier may be issued in fiscal 2021 for the second supplier contract to start in 2023.”Mark that date in your calendars, folks. It could be the date that AeroVironment loses its Switchblade monopoly.