Same reason that you get a shock from the hose. Static build up in the plastic parts.
A grounding strap on the hose to the block would have prevented this. Same as when working on electronics, always use a grounding strap from yourself to the parts.
Don't think I posted this yet, but I ended up getting a 0.100" spacer and two 0.020 viton coated Cometic gaskets(total of 0.140"). Looks like everything is going to work flawlessly, will be doing a cold compression check in a few days when I drop the engine into the chassis.