Without actually seeing the circuit it's hard to say.
I suspect that the unit uses an isolation transformer as part of it's surge protection (hence "advanced") and when you turn it off -at that switch- it's sending a voltage spike back down the line because of the sudden simultaneous removal of all the loads.
It's a characteristic of transformers and coils known as "inductive kick".
No, a surge protector shouldn't do that but one of a marginal design might, and Belkin is definitely 'consumer grade' equipment.
There should be circuits/parts to absorb the inductive kick but low end gear tends to skimp on parts and they may have skimped a little bit too much on this one.
[It may also just be a faulty unit.]
I would exchange the Belkin and see if the problem goes away.
If not then in the future shut off each device manually (at least the PC) -then- open the switch on the Belkin. You shouldn't have any problems opening the switch on the Belkin when it's not loaded.
