I believe it's the condo boom that has spurred the boom of new office buildings as businesses want to locate in locations convenient for their employees and clients to access. Office developers are using land more efficiently (taller buildings) due to the limited space available. It's a great thing this Iain Dobson wasn't King of Toronto since he probably would have halted all new condo developments as he waited, for a very long time, for offices buildings to rise.
A little off topic, but Scarborough Center is a wasteland. Planners back in the day masterplanned an auto-oriented community and reserved significant amounts of land for office use. That didn't work out to well. Hopefully Scarborough's "master planned downtown" will be a lesson learnt to guard against the next Iain Dobson that advocates keeping land vacant near subway stations so office buildings will magically appear.