What do the dead parts look like? If they're soft and squishy, then you could have rot due to overwatering and/or infection; if they're dry and hard, then it's more likely to be due to lack of water, or it could be due to an insect infestation - mine had an attack of whitefly (which is quite hard to spot on petunias because they're naturally sticky) and started to die off similarly, but they recovered after a spray.
What do the dead parts look like? If they're soft and squishy, then you could have rot due to overwatering and/or infection; if they're dry and hard, then it's more likely to be due to lack of water, or it could be due to an insect infestation - mine had an attack of whitefly (which is quite hard to spot on petunias because they're naturally sticky) and started to die off similarly, but they recovered after a spray.