iPad exploded
Any ideas on what caused this? (It was not dropped.)
Rather than engage in speculation, you would be best advised to directly contact Apple Support; you’ll find the Contact Support link at top-right of this page.
Alternatively, please take the corpse along to the Genius Bar at your local Apple Store - where appropriate investigation can be undertaken by Apple Technicians. Whilst the iPad is unrepairable, the cause can be established and appropriate advice/remediation offered.