What is runaway excitation?
When I have used (or seen) this term, it usually means that recurrent excitatory connections in a network have become stronger than the inhibitory feedback. This means that excitatory activity will grow stronger and stronger, without inhibitory influences to keep it in check. Eventually, the activity within the network will be limited only by spike-rate saturation. This type of network configuration is sometimes used as a model for an epileptic seizure.