I've had at least a dozen iPads & iPad Minis do this in Launchport sleeves and also the newer iPort surface mount docks.  Three of them had puffed out so badly that the glass on the front has popped off and separated from the body of the iPad.

Initially I thought it might be an issue with a bad production run of batteries, but now I've seen it happen over 3 years to several different models of iPads that were purchased years apart.

Now I think it's probably a combination of the frail nature of LiPo batteries coupled with being on the charger for long periods of time along with the additional heat created from the LaunchPort sleeve/inductive charging plus keeping the screens lit up for long durations.  I'd hoped the new surface mount units with PoE power wouldn't have the same problem, but I've seen the same thing happen with them over the last 12-16 months.

A few iPads lasted less than a year and were replaced under warranty... a few were just out of warranty by 6-8 weeks and were replaced anyway.

If your units are well out of warranty, Apple will repair/replace the iPad for a flat rate of $100.  I've done this for 6-8 units and they have all been replaced with what looks like a new device.

You can setup the repair online by providing the devices serial number and they'll even send you a box and packing materials/label.

I've spoken with a few people at iPort and they state that they are aware of the issue and pass the blame to Apple stating their docks have passed Apples "Made for iPad" (MFi) certification so that it's simple an Apple/Battery issue.