The simple answer is because you are using shared home and limit switches on your z axis, the vibration or movement is triggering the limit switch.... easy option would be in install a separate limit switch for the z axis....
The other way to do it would be to have a relay via output break the input before the clamp closes... you would probably set something up like a custom clamp macro....
Within which you would check the z axis DRO, and if it was less than 0.5", deactivatesignal(output1), then apply clamp, activatesignal(output2), where op1 is your new relay, and output2 is your pneumatic clamp.
Which you choose all depends on if you have a spare input or output.
Software wise, you would need to write a macro which same as the above one, would check the z axis dro, and if closer than 1/2" say, would toggle override limits DoOemButton (150).... the problem with this is it would override all limits and is not specific by axis (if you've a limit switch on the other chop saw axis).... then when the clamp releases, toggle the override limits off, if getled(34) is active.
Hence you'd need two macros to activate your clamp and release the clamp (not sure how you are doing them now... m7/m9?)