These trade unions are notorious for that. I worked as a labor legal advisor and especially the unions for temporary employment agency start 'barking' and demand loads of money (even from years back). Sometimes it's not even clear which union is applicable.

You probably have all the info right now, but make sure everything is 'in line'. I mean, have your company codes at the tax authority match the applicable union match the actual things that your company does. Depending on the jobs of the employees, it might be smart to split the company into multiple legal entities.

All in all you can be happy that this happened within a couple of months. Finding this out when you're years underway and then having to pay millions... I've seen plenty of these cases.

Want to start a business in The Netherlands? Make sure to do a 'CAO check' first, think about how to structure your company (one entity? multiple entities? what job goes where?), and do these checks again once you pivot or make certain changes to the actual work that your company does.

The rationale for this is also pretty simple: somebody got to pay for all this nice social security. They say it's part of the risk of being an entrepreneur.