Here's one possible explanation for Hegseth calling so many top generals and admirals to a meeting on September 30th. I hope this is far fetched, but here's my hypothetical scenario.

The officers will be met by thousands of heavily armed ICE goons who will immediately arrest and disappear them. They will then be replaced by junior officers loyal to Trump. In other words, an even larger scale, and possibly less bloody, Night of the Long Knives.