Why an object became invalid in oracle databse. There are some invalid views and functions in my database. 

I am not sure what changes caused these objects to became invalid since many people are working on the same database.

How can I see which change caused the view to become invalid?
In short, no.

You can however use the DBA_DEPENDENCIES view to check which objects the views and functions depend on, then query DBA_OBJECTS to check the last_ddl_time for each of these dependant objects - that should give you an indication of which objects have changed and caused the view to be invalid as a result.

It's quite normal for objects to become invalid in development databases when columns are added to tables etc.