Why can nervous inflammation radiate symmetrically in the limbs?
Padua et al. (1998) did find that 87% of followed patients developed bilateral CTS (i.e., carpal tunnel syndrome in both hands). 


Most patients that develop unilateral symptoms, eventually have the symptoms spread to the contralateral (opposite) arm. 


Bagatur &amp; Zorer (2011) found that 59% of patients they followed reported bilateral symptoms initially, while 73% of remaining patients had symptoms eventually develop in the contralateral arm (even after receiving surgery on the first!)



So why? From what I could gather, we don't really know. From Bagatur &amp; Zorer (2011):


  we assume that bilaterality may be time-dependent... [but] a definite conclusion could not be drawn.


However, research regarding "double crush syndrome" suggests that perhaps issues closer to the cervical spinal cord may play some role:


Pierre-Jerome &amp; Bekkelund (2003) found that cervical intervertebral foramina were narrower in observed patients and affected nerve roots.


Although their research provided evidence for ipsilateral double crush symptoms, their study provides credence to the fact that more cervically-oriented issues (often which should show up bilaterally) could lead to CTS symptoms. 



Not the greatest of answers, but a thorough search really didin;t turn up anything more recent or helpful. I'll update if I find additional info.



Citations


Bagatur, A. E., &amp; Zorer, G. (2001). The carpal tunnel syndrome is a bilateral disorder. Bone &amp; Joint Journal, 83(5), 655-658.
Padua, L., Padua, R., Nazzaro, M., &amp; Tonali, P. (1998). Incidence of bilateral symptoms in carpal tunnel syndrome. The Journal of Hand Surgery: British &amp; European Volume, 23(5), 603-606.
Pierre-Jerome, C., Bekkelund, S. I., Pierre-Jerome, C., &amp; Bekkelund, S. I. (2003). Magnetic resonance assessment of the double-crush phenomenon in patients with carpal tunnel syndrome: a bilateral quantitative study. Scandinavian journal of plastic and reconstructive surgery and hand surgery, 37(1), 46-53.