Under normal circumstances, yes, the installer should start on its own.
My first thought is, as I've seen quite a few people do this on Mastodon (and I've done this myself once by accident!), if you had flashed it to a partition rather than the whole disk? (i.e. on Linux flashing to /dev/sdb1 rather than /dev/sdb)
I've also flashed amd64 install media for a machine that would run the i386 port of NetBSD, so pmjv's question is also worth looking into.