I’ve updated to the latest 14145 pre-release, from a slightly older build (don’t remember which exactly). Before the update, I had multiple PDF files opened on multiple Sumatra windows. With this new build, Sumatra loads the files slowly (used to be fast), becoming slower and less responsive with each loaded file. After a few are loaded, the program stops responding, and eventually killed by Windows.
Down-grading to the latest stable version 3.3.3 solves the problem.