Galaxy S4 back button pressing itself
It appears to be a strictly mechanical issue. If the phone is bent in any way, it can put pressure on the digitizer without any involvement from you.  If you take the phone apart again (but don't disconnect the cables), does the digitizer behave the same or was the pressure from the frame causing the issue?   This is obviously not a specific answer to your question, but answers to these questions would help you understand the next steps to take.  Issues like this are usually either a faulty digitizer or faulty (or badly installed) connectors.