https://mfn71.myfootballnow.com/gamecenter/view/108#23390
https://mfn71.myfootballnow.com/gamecenter/view/108#23408

On returns, a defender need not be anywhere near him but if it's perpendicular, it seems to equal no return, and a fair catch. The second one was even worse, he had blockers galore in front of him.

Any idea why this is happening?

1st one i'm guessing called fair catch as the gunner was approaching which is realistic
2nd one was a kickoff into the end zone in the 1st quarter, you will rarely see a team run out of the end zone in this situation

Kickoff touchback as a non-zero probability if the ball is caught in the end zone at all, the deeper the catch the higher the probability.