Can an employer withdraw from a multiemployer plan for any or no reason at all? Is the ability to withdraw typically addressed in the CBA?

It would have to be negotiated with the Union, since inclusion in Multiemployer plans is normally a bargained for benefit.

Good luck getting out.

This may be stating the obvious, but keep in mind that many multiemployer pension funds have significant unfunded liability. While the employer may be able to walk away from the union upon the expiration of the collective bargaining agreement, the withdrawal liability assessment could be a financial nightmare for the employer.