ive been reading up about urine (exciting i know) and found this

Urine contains large amounts of urea, an excellent source of nitrogen for plants. As such it is a useful accelerator for compost. Urea is 10,000 times less toxic than ammonia and is a byproduct of deamination (2 NH3 molecules) and cellular respiration's (1 CO2 molecule) products combining together. Other components include various inorganic salts such as sodium chloride
so if the nitrogen in urine is used as an accelerator for compost, whats it doing to bamboo/cotton etc.... perhaps it isnt been washed out properly and combined with heat (washing/dryer/sun) its speeding up the 'decomposition' of your nappies just a thought!!