Mutation mode ''Defence''?

Sometimes when i claw at the humans ..like today this spop was shooting at me i managed to get close i clawed her then i heard that sound when u get them but instead of mutating i got icon saying ''Defence'' idk wat it is or how u get it,, the picture below was a few days ago was in pile of humans clawing around got ''Defence'' first swing then got mutated.. is this that zp item armor where lest u get clawed once and not mutate?
zmk0hu.jpg

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