Agility has nothing to do with software quality.
Remember project management triangle: time, scope, quality.
The more time you put into development or testing, the less bugs you may find.
The less scopes you push into the application, the less bugs you may get.
In agile, you need to decide the least scope with the least time to develop an acceptable quality of working product. New changes introduce a new complexity to the software, and devs need time to formulate the right complexity. You need to make a balance between time or scope, or the complexity of the software will produce either less testable or less readable codes, which leads to more buggy app.
If the agile approach seemed to be cause of it, then the agile process must have been conducted improperly.