Seems as though someone in the Windows 10 team may have jumped the gun on pressing the release button for Windows 10 and Server builds 1903.
According to a report from ZDNet and corroborated by several MSDN members, Microsoft has been seeding its Windows 10 1903 update to developers early.
For the past few months, Insiders have come to recognize the Windows 10 1903 build as Microsoft's official May Windows 10 Update, and under that presumption many believed the release of the update would occur sometime in May.
Furthermore, Microsoft officials reiterated the notion that consumers and businesses could expect to see the May Update rollout in late May to coincide with an 18-month servicing plan as a means to secure a qualitative improvement in user experience, however, it seems as though something or someone within Microsoft decided sooner is better.
It now appears that Windows 10 build 18362.30 and Windows Server 1903 are now readily available for download on the MSDN servers with the Windows 1903 bits still waiting to hit the VLSC and Evaluation Center.
When contacted about the subtle release, Microsoft had no official response but author Mary Jo Foley does highlight the possible intentionality of the rollout as MSDN does stand for Microsoft Developer Network and that the company is confident enough in the build to let developers have an early crack at it.
Typically, the Windows update rollouts for the MSDN channel occur roughly around the same time as other business channels, but there seems to be a shift in how the Windows team is assessing the May Update and appears to be giving developers a month head start this time around.