In multilevel mode, two or more classification levels of data exist, some people are not cleared for all the data on the system.
Risk is higher because sensitive data could be made available to someone not validated as being capable of maintaining secrecy of that data (i.e., not cleared for it).
In other security modes, all users have the necessary clearance for all data on the system. Source: LaROSA, Jeanette (domain leader), Application and System Development Security CISSP Open Study Guide, version 3.0, January 2002.