Subject: What leads to identical twins having non-identical fingerprints?

After scouring the MadSci archives, I have become fascinated by the formation 
of fingerprints.  One important question, however, remains distinctly 
unanswered:

Monozygotic (identical; MZ) twins have (essentially) identical DNA, and are 
therefore identical in all respects, except where their DNA is later "cut and 
pasted" (e.g. immune function/MHC) or where environmental factors exert their 
influence.

As MZ twin DNA is presumably identical at the loci coding for fingerprints, I 
can only conclude that there are environmental factors that lead to identical 
twins having absolutely non-identical fingerprints.

Are the environmental factor(s) that influence fingerprint formation known?

Many thanks for putting my mind at rest (and settling a bet!).

Re: What leads to identical twins having non-identical fingerprints?