Typical transformer design lifes are 20 to 40 years because they are in near continuous operation (A random source). With the downtown core increasingly being built up, the utility did put in more capacity but are also relying on the existing aging equipment to take on more load.
Things naturally wear, weather, corrode over time, even things installed underground or in indoor vaults. Raccoon-shaped wires also strain equipment across the network, beyond the immediate failure point. There are other cases of “uniquely Toronto” problems across downtown and the 6 amalgamated boroughs that joined in 1999.