Kenneth Fisher points out that there are defaults when altering columns:
So here is the thing. When you change one you change them all. That means if you don’t specify a precision when you can then you get the default. That’s not exactly a common problem though. Usually what you are changing is the precision (or possibly the datatype). What is a common mistake is not specifying the nullability.
When modifying DDL, make sure that you keep it consistent and complete.