Metastability comes in when you think about the problem that originally triggered congestion. A normal network overload caused by a bulk transfer, for example, can be fixed by canceling the transfer. Once you add the feedback loop, however, you need to both cancel the transfer and remove lots of other load. I use the term metastable because although it is fully stable (that's the problem!) it isn't the "ground state". Another way to say it is that the metastable states are only local attractors.