Death grip does not mean one holding the phone really tight. The term was made popular by iphone 4's death of signal problem by gripping the phone in a certain way. Mr. Jobs offered free cases to people having the problem back then.
Maybe you can take your HTC One out of the Otterbox and see if you have the same problem. The YouTube video I quoted earlier would show you how to try to reproduce the problem:
Sprint HTC One death grip problem - YouTube
Please post your finding. Thank you.