It's possible it's QWERTY-related as a sibling suggests, but there's also precedent for this specific kind of sound change in many dialects of English: /d/ is a stop consonant, which are commonly inserted in the middle of a nasal (/n/) and fricative (/f/), and in particular /d/ is produced with the tip of the tongue the same way that /n/ is, so it and /t/ (the unvoiced variant) are the most likely stop consonants to be inserted after /n/.

Other examples of this effect:

Samson -> Sampson

prince -> prints

hamster -> hampster

warmth -> warmpth

fence -> fents