Clonus and hyper-reflexia sounds spinal in origin. As heavy lifting is involved, is there a possibility of microvascular spinal infarct as a result of increased intra-abdominal pressure? There have been reports in the literature of transverse myelitis presentations following heavy lifting, later assumed to be microvascular infarct. Are there any other spinal symptoms and what do the Wikipedia reference-linkMRI results suggest? As to the impact of the freezing temperatures; perhaps the combination of redirected blood flow and increased abdominal pressure may have been contributory, or perhaps it may have something to do with the demographics of those who chose to be freezer workers?... Your guess is as good as mine.