Did I completely miss something, or did they really not explain who was behind the terrorist bombings in the film 'Brazil'? At first it's suspected to be the female truck driver, but then it is revealed her deliveries were harmless. So what's the deal?
No answer is provided. They seem to be connected to the vaguely defined "terrorists", but we can't even be sure if the terrorists exist. Both Jill Layton and Harry Tuttle are likely considered to be terrorists by the government, but only because it's a convenient term. Neither are ever linked to anyone else who could be considered a terrorist, and in particular neither is ever linked to any bombings.
There might be an actual terrorist group of some kind, but the movie isn't about them. If they exist, they're part of the background of Sam Lowry's world. The bombings might also be staged by the government to justify their own actions, but again that's never explained. It's hard to believe that the government in the film would be competent enough to pull off such an operation.
In the context of the film, the most plausible explanation may be that there are no bombings, but that there are sometimes explosions due to poor maintenance, for example of gas lines. Considering the early line that 'they've gone back to metric without telling us", it wouldn't be surprising if things blew up sometimes due to the extreme bureaucratic incompetence on display in the film. Or consider the guys who show up to "fix" Sam's apartment-- if there are gas lines in Sam's building, how likely are they to treat them safely? The explosions could be blamed on "terrorists" as a convenient cover story.