Not enough water - it's not likely to get more in the ground, in fact, they're easier to keep watered properly in a pot. Just because its raining doesn't mean the rootball of your plant has got thoroughly wet, and when you plant a tree or shrub, they will need regular watering during dryer spells for the first two seasons, until they establish their own water seeking roots. It may be that you're not watering it properly, or sufficiently either in a pot or in the ground - always water early morning or at the end of the day, with a can with the rose off, applying the water slowly to the compost or ground around the stems - an Acer in a largish pot during dry spells will need about a gallon every few days.