A voltage spike, static electricty, or maybe just pure coincidence. I had a new member of staff a few years back who was so prone to accumulating large static charges (wrist straps et al attached) that in the end we had to let her go.
Even with surge protection, man made fibre in clothing can cause this on quite a dramatic scale.
This poor girl could actually generate a visible blue spark! Scary for me with visions of ££££ worth of kit being rendered useless. She now works in the insurance field, but noe with computers!