Too early to say what caused yesterday’s failures.
The OLE is a complex system, and there are many potential points of failure. Even something as simple as a dropper becoming detached (the thin vertical wires between the contact wire and catenary wire) can cause a dewirement if the circumstances are right (or wrong, depending on your point of view!)
However what does tend to happen in very hot weather is that the whole system gets a bit ‘looser’; whilst most OLE is tensioned, it is only the contact and catenary wires that get the tension - the rest doesn’t (headspans, cross contact wires, etc etc). This all means that certain types of defect become more pronounced, and more likely to cause an issue than in ‘normal’ temperatures. There’s a similar effect in very cold weather, where the whole system tightens up.
Clearly in an ideal world there would be no defects in the OLE, but there are always some.