If an entry in an expense voucher is the same as in an invoice/bill, tax is automatically added to the expense voucher entry. 
When the first entry in an expense voucher adds tax automatically, this is also done to remaining entries in the same voucher. 

Example:
Vendor Bill is created with line item: Food + tax table reference, posted to account Exp:Marketing:Dining. Let's say the tax table specifies 15% tax and the line item is 100$ - total invoice becomes 115$ - all correct and well so far. 

Expense voucher created on an employee, with the same Line item = Food. Tax is then calculated on all items in the voucher.

Probable cause:
Due to that GNUCash remembers the previous line item (=Food) it prefills the other field in the entry - my suspicion also the Tax table field which is not visible on the Expense Voucher line item Entry form.

Additional line items on the Entry form inherits this tax table settings. 

Removing all entries on the Expense Voucher will not reset this hidden tax table link. Differences in the XML database between normal Voucher entries and the entries with tax has this XML tag  <entry:b-taxtable type="guid">

Regards, Harald