It's the fact that they burn the beans to increase the caffine content. Hyper caffiniated coffee may have cropped up everywhere in the US, and with Espresso available on every street corner, it's a little less special.
But Timmy's is still the most caffine that you can get in a coffee for under $2.
It's a popular misconception that Espresso and other dark roasted coffee's have more caffeine. In reality, they contain far less caffeine then the lighter roasts because the roasting process caramelizes the caffeine. A cup of Espresso contains about 40mg of caffeine compared to the stuff you brew at home which contains 85mg.
And as for Timmy's being the most caffeine you can get for under $2. Its cheaper to invest in a good thermos and brew your own...not that I don't enjoy a good Tim's coffee every so often but my coffee consumption could put me in the poor house if I didn't make the majority myself.