My iPad Air 2 gets warm at the back while charging why?

“When using the device or charging the battery, it is normal for it to get warm. The exterior of the device functions as a cooling surface that transfers heat from inside the device to the cooler air outside.”

Source: Keeping iPhone, iPad, and iPod touch within acceptable operating temperatures - Apple Support

Hi..

Warm is ok but if it actually feels hot to the touch, make an appointment at a Genius Bar

Or send your iPad in for service > iPad Other Repairs