I believe the Lithium Ion Batteries are susceptible to some sort of thermal heating problem when they have been overcharged, hence fire. Boeing say there are systems in place but clearly this isn't working. Either that or the manufacture of these batteries got something wrong here, this particular manufacture is Japanese. Lithium Ion batteries have not been used to this extent in an airliner before, and the A350 will have these batteries to a similar extent to the B787. Hence the A350 could have similar problems, although they have a different manufacturer and the benefit of seeing Boeing's problems.
It is also worth noting that Lithium Ion batteries are not causing problems just for Boeing, with new electrical vehicles they are causing problems for manufacturers of these as well. Lithium Ion batteries are best kept small with many safeguards to prevent overcharging. I think putting something like a Lithium Ion battery in is a little stupid (even if they are half the weight and double the energy of Nickel-Metal Hydride) and I personally feel Boeing and potentially Airbus have taken a risk to far in terms of safety with the batteries. The worst bit about Lithium Ion batteries where aviation is concerned is the fact that once they start burning they produce their own oxygen to keep the fire burning, which is dangerous in the air, especially seen as these batteries in the 787 are quite large by normal standards of Lithium Ion Batteries. It could mean a fire burns out of control before the aircraft can land.
In my view it is either manufacturing fault, not unheard of, or Boeing have screwed up in the choice of batteries and safeguards for their aircraft. I also feel potentially Airbus have screwed up as well with battery choice, so I wouldn't be surprised to see the A350 running into problems.
The other stuff I think is the least important stuff, that can be sorted easily, or poor maintenance, it does seem to be mostly happening to Japanese Airlines!