maspushagent using CPU
The maspushagent is the "push agent" for the "Mac App Store". Being a "push agent" means that it used for receiving/processing notifications from Apple's servers. It could for example be notifications about an update.

The program is automatically started by launchd when a notification is received by the Mac destined for the Mac App Store.

Therefore it should be safe to simply kill the mapushagent if it is behaving like you describe. Hopefully that fixes the issue for you.