[Sansevierias] Irregular leaf growth in sansevierias?

Sometimes, leaves on my sansevierias grow in irregular forms where they
seem to get pinched and crinkled and grow narrower like an hourglass shape,
if you will. I have noticed this in my S. liberica and S. trifasciata
'Moonshine.'

My guess is that it might be 'cultural,' maybe has to do with water quality
or amount, or irregularities in photoperiod or photo intensity,
temperature, or drafts perhaps? I am growing them indoors. Any ideas?
Thanks!