This is true, but it doesn't stem from union organisers opportunistically seizing the best moment to negotiate. Rather, some people are always trying to organise collective action, management usually calls their bluff, and the striking employees end up coming back to work or being replaced. When they can't be replaced easily or they have alternatives to coming back to work, management is forced to the negotiating table.

There's an alternative narrative where the government steps in and says hey, you have to allow unions. In practice, while some government support is necessary, this doesn't seem to lead to powerful unions in the same way.