SIMP-0136 is a T2.5 UCD whose youth and low mass make it an ideal analogue for the giant, long period exoplanet population. It is variable in both the infrared and the radio, which has been attributed to changes in the cloud coverage and the presence of an aurora respectively. We obtained time-series spectra of SIMP-0136 covering one full rotation with both NIRSpec/PRISM and MIRI/LRS, and performed a series of time-resolved atmospheric retrievals using petitRADTRANS to measure changes in the thermal structure, chemistry, and cloudiness. We inferred the presence of a 250 K thermal inversion above 10 mbar, and propose that this is due to the deposition of energy into the upper atmosphere by an aurora. The primary driver of the variability was found to be changes in the temperature profile at pressures deeper than 10 mbar, which resulted in variation of Teff by 5 K. Patchy silicate clouds were required to fit the spectra, but the cloud properties were not found to systematically vary with phase. This work paints a portrait of an L/T transition object whose primary variability mechanisms are magnetic and thermodynamic in nature, rather than due to inhomogeneous cloud coverage.