Disclaimer: Not my area of research
Since IAA (auxins) can induce growth, partially acting through cell elongation (http://www.biosynth.com/index.asp?topic_id=139), it seems likely that the explanation to your result is differential growth/elongation rates on the upper and lower side of the leaves. This could result from uneven spraying, e.g. by spraying the leaves from the top. Faster growth on the top part of the leaf would lead to downward twisting, which seems to be what is happening most of the time.
You could test if this is the case by doing a simple experiment where you carefully apply IAA with a brush on both sides of the leaves for some plants, while others are sprayed. If uneven application is the cause you should see twisting when spraying but not when applying with a brush.
However, auxin is toxic to plants at high concentrations, so you also need to consider the concentration you are using.