Song Jiang's paper on [http://www.c3.lanl.gov/~sjiang/pubs/papers/jiang05:token.pdf Simple token based thrashing protection] has an excellent description on SystemThrashing:

"Whenever page faults occur due to memory shortage in a multiprogramming environment, false LRU pages of a program can be generated, which will weaken the ability of the program to achieve its working set. For example, if a program does not access the already obtained memory pages on the false LRU condition, these pages may become replacement candidates (LRUpages) when the memory space is being demanded by other interacting programs. When the program is ready to use these pages in its execution turn, these LRU pages may have been replaced to satisfy requested allocations of other programs. The program then has to ask the virtual memory system to retrieve these pages by replacing LRU pages of
others, possibly generating false LRU pages for other programs. The false LRU pages may be cascaded among the interacting programs, eventually causing system thrashing."