To respond by losing one's sense of perspective and becoming agitated in the extreme.
Possibly borrowed from meteorology. A cellular storm "goes linear" when it turns into more of a squall line (or merges with an existing storm) -- i.e., that it's going to turn into a big black line of death. (from stormtrack.org).
I have a sense that there should also be a nuclear physics metaphor, but I don't know it.
My boss went linear when I showed him my expense report.