An .APK (archive) is a .ZIP (archive) file, just the extension name differs. .APKs also have the same first 2 bytes "PK" as .ZIPs (PK=Phil Katz, IIRC, the original developer of 'Zip' archives)
Looks like maybe the client is somehow 'opening' the APK (which is opening as a ZIP) rather than 'Installing' the APK.