Likely drought and/or prolonged exposure to hot sun - water more, or post a photograph to show symptoms.

Pieris 'Little Heath' expands its root system like you wouldn't believe - it may be so pot-bound that it's impossible to effectively water it. Tip it out of its pot to see if that's the case.
I had to pot mine on several times in a very short space of time because of the rapid expansion. It eventually got to the stage where no pot was big enough, and it needed to be in the ground.
However, it never got to the stage where leaves were turning brown and dropping off. Pieris likes a soil which is on the acid side, so, if it isn't pot-bound or lacking water, the compost you've used may not be to its liking.