The explanation I read online is that Translink (the transport authority) lost one of the 2 rail grinders they contracted, and had to buy a grinder for use. So for a few years, while waiting for the grinder to arrive, the rails were not getting the regular grinding needed that'd lessen the wheels-on-rail noise.
An unverified rumor I heard was that Translink cheaped out and bought a batch of substandard rails that are softer than needed. This means the rails wear out and become out of spec faster. The issue was compounded by the fact that the softer rails were used to replace a large portions of routes with sharp curves.
The rolling stocks use linear induction propulsion, which means there is no electric motor on the train. The trains are very light as a result and require different maintenance.