The obligation was never voided and reversed after the check to Vendor X was voided. The vendor was then changed on the obligation to Vendor Y, and as a result, the rest of the steps (the void from Vendor X and reissue to Vendor Y) appear on Vendor Y's record.
The reason why this occurred is historically the system is set to not allow the Vendor number to be changed once a check is written; however, prior to Version 7.6 you were able to change the vendor number if the original check was voided but the obligation was not. In Version 7.60 there is a flag in AP that the user can set to not allow a change to the obligation past a certain stage.
Note, simply changing the vendor number on an invoice will not post the change to the obligation in General Ledger. If a check is processed for the incorrect vendor, both the check and the original obligation will need to be voided.