I doubt if this has anything to do with property tax.
Homeownersâ€™ associations have the ability to levy assessments on the home owners for extraordinary expenses. Major unbudgeted, unplanned repairs could be a cause. I donâ€™t know the details in this situation. Your agent can obtain the information from the listing agent for the property.
If you buy, your property tax will be based on the purchase price which would normally be the new assessed value. You would normally not agree to assume back property taxes that the seller did not pay.