According to TEPCO's handout for the press for June 18, TEPCO stopped the entire water processing system manually when the surface radiation at the "skid" for removing oil and technetium exceeded 4 millisievert/hour, the limit set for replacing the zeolite cylinders (which TEPCO calls "vessel") inside the skid.
That "skid" contains 4 cylinders filled with SMZ (surfactant modified zeolite) to absorb oil and technetium. (The system diagram by TEPCO is provided below, with English labels added by me.)
The cause for the high radiation at the oil-technetium skid wasn't given in the handout, but in the press conference TEPCO listed the following as potential cause:
The radiation measurement device picked up the radiation from the pipe that transports highly contaminated water to the system; or
The water contained extremely radioactive sludge; or
The water contained too much seawater for the system to handle.
By the way, the skids were welded and fabricated by a company in Delta County, Colorado, called Industrial Systems, Inc.. According to the Delta County Independent's article on June 18, 2011, ISI has tripled the number of workers - welders, fabricators, machinists - to 75 to work 24/7 on the project, which for them now includes internal components for filter assemblies in addition to the skids.