Xcode.app gets scanned by socketfilterfw (OS X Firewall) at every launch
Xcode is likely trying to use the network. This is triggering a rule in your firewall that results in Xcode's code signature being checked – requiring a read of the complete application bundle.

Experiment with the following option in OS X's firewall settings:


System Preferences.app > 
Security &amp; Privacy >
Firewall >
Firewall Options… >
Automatically allow signed software to receive incoming connections


I have this option disabled. As a result, I am frequently prompted to allow Xcode incoming network access. I always deny access as doing so still allows local connections between Xcode and underlying development tools.

As a side note, I am unable to reproduce the behaviour you are seeing on OS X 10.10.3 and Xcode 6.3.2.

Alternative Firewalls

If you do decide to disable your Mac's built-in firewall, investigate using a third party alternative such as Little Snitch:  Reasons to prefer Little Snitch over the built-in firewall