SQL Server: Why would an ALTER TABLE to add nullable column run forever? SQL Server 2014 Std Edition

We add columns all the time, and the ALTER TABLE commands return instantly.

E.g.

alter table [dbo].[Account] add ChildAccount nvarchar(100) NULL
GO 


But at the moment, this alter table seems to run forever.

What would cause this?
Yes, some other session was blocking. A few min later, this command executed instantly.