Being hard to open doesn’t actually mean there is net charge on a sleeve. When it is difficult to open it up it’s because when the two sides are pulled apart electrons are being torn off one surface more than the other. The other becomes positively charged. Positive and negative charges attract.
I don’t know why this happens, but it affects the entire pack usually especially when first opened. I expect humidity affects it, and time should help the neutralisation of charge imbalance.
Phil