It seems the cause of the problem is Core 0.12 nodes and miners not relaying valid transactions, causing the mempool to expand. The net result is less nodes and nodes that don't switch to the centralized control of Core 0.12 will become overloaded contributing to the problem.
It is highly unlikely 0.12 has anything to do with it, as it represents just 15% of the network. If a tx is not relayed it will be rebroadcast until relayed. However it will stay in the mempool as unprocessed if it doesn't have the right fees.