What are the possible causes of sudden extremely high internal resistances in Lithium batteries?
The protection measures against thermal runaway in some Li-ion batteries include


Thermal shutdown separator, pore closure


See Failure Modes &amp; Effects Criticality Analysis of Lithium-Ion Battery Electric and Plug-in Hybrid Vehicles

The cells can also contain


CID or pressure valve, will disable the cell permanently if the pressure is to high in the cell 


See The Anatomy of a Protected LiIon Battery

Those 18650 cells also contain a protection circuit at the negative end.

Protection circuits are likely to include protection for


Excessive temperatures will cause all cells to fail eventually. Most protection circuits therefore incorporate a thermal fuse which will permanently shut down the battery if its temperature exceeds a predetermined limit.


See Lithium Battery Failures