Probably nothing... you may have just run out of memory... A 100 MB model on a 32 bit system will take up quite a lot of RAM (check how much it's taking in TaskManager), and over about 1.4-1.6 Gigs of RAM usage, Rhino will crash on a 32 bit system without the 3Gb switch set.
Other things to check:
See if you have any bad objects in the file. If so fix them.
Save the file, close and re-open it. This may free up some memory if something went wrong somewhere. Using ClearUndo after you have been working for awhile if you want to clear the Undo memory stack (you will lose your undos though).
Another thing is to try to reduce the memory consumption by doing things piecewise. Is the model you're splitting all one piece? If not, you could try splitting it part by part instead of all at once. Save after each operation and clear undo if necessary.
What tool are you splitting with? Solid booleans will take up more memory and calc time than a simple split or trim. So as a last resort, you can use ExtractSrf to extract all the surfaces around your split plane, split those, then rejoin the parts back together and Cap the (now open) ends...
As a last resort,