A padlock can be the result of several things.
If you have Force Order on, padlocks will appear for any module that is not yet available to the user.
Additionally, if a module has This Item is Mandatory For Course Completion enabled, all following modules will appear locked until a user finishes it.
Lastly, if you have a Module Start Date enabled that is in the future, the padlock, as well as a calendar icon, will appear.
If you look into your settings for your course and/or affected modules, you should be able to identify what is creating the padlock.