There is a learning ladder for RIT scores here:
http://www.powayusd.com/projects/edtechcentral/lladder/Default.asp

If she's not being exposed to material at her readiness level at school then I would look at trying to change that or after schooling. Here's a website with Math activities by RIT level:
http://www.sowashco.k12.mn.us/ro/Pages/studentlinks/map/

Here's information about negative growth on the MAP test:
http://www.nwea.org/support/article/554
You may want to ask your daughter if any of it applies in her case. It is just one test and a quick snapshot at that. Do you have any other norm referenced tests to compare? Like you, if I saw that kind of drop in percentiles I'd be asking questions.