Believe what you want about these bombs, they are actually pretty easy to put together. I was nearly killed by a similar device in a post office. One Mormon in Provo in a dispute with another Mormon over a woman and he built your basic shotgun shell/nail bomb and sent it through the mail. It didn't completely work ~ so here I and others are today ~
The big deal with a pressure cooker is the additional time alloted to the black powder to burn ~ that provides even more pressure ~ and hence, a small firework launcher becomes a fairly powerful bomb.
All of this reminds me of an old friend who used to work for a chemical company manufacturing high explosives on site ~ for mining down in Chile. One day the union told the company to do so and so and the army told the company to do something else. He left on the first plane out! We didn't have the internet in those days, but he said all the materials on bombmaking were all pretty much true ~ but be careful with the fusing.
Remember the way the boys walked in that FBI pastiche of cellphone pictures? They were avoiding exposure to the sensor on the triggering device until they'd set the bombs. They didn't want to get blown up.
This wasn't a suicide bombing job ~ probably a commercial venture ~ which means we don't really know who is behind it yet.
The effect is actually multiplicative: the pressure build-up actually makes the chemical reaction go faster, which in turn intensifies the pressure build-up. So, it all happens really fast.