As you can tell from my blog, I have had many issues with my FAST search service. This time I ran into an issue where the fixmlindex.exe process was runaway. This memory leak caused issues with my SSRS service. It doesn't directly impact SSRS, it's just that for me I have these services on the same app server in my SharePoint 2010 farm. So when this process ran away, it affected my SSRS instance which also uses a ton of memory. FAST and SSRS were competing for whatever memory they could get.
Fixing this issue isn't too bad. All you need to do is stop and start FAST. Use the process I outlined in my post Properly Stop and Start FAST Search for SharePoint 2010.