Turn 6, my opponent plays Enduring Courage on an empty board.

I kill it on my turn and pass.

Turn 7, my opponent plays Threefold Thunderhulk and before it makes its tokens it spawns a copy of itself. Huh?

Setting aside that weirdness, the expectation is that 10 tokens would be generated (5 per thunderhulk) and that many more would spawn after attacks were declared.

Instead, they continued to spawn WHILE declaring attacks, including while tapping the tokens from the ETB triggers. How?