Why do they keep accusing people of being the bomber? Why is Angela accused?

I believe that the chapter in question is chapter 21. This is the chapter in which the judge begins to suspect that Angela Wexler might be the bomber. In addition to suspecting her, the judge actually believes there is some merit in the idea, too. The judge begins to suspect Angela as a legitimate culprit for the bombings because Angela is someone that everybody immediately considers as an impossibility for such a crime. Everybody just sees her beauty, and they all assume that someone as pretty and sweet as her could not possibly be the bomber.

Angela could not be the bomber, not that sweet, pretty thing. Thing? Is that how she regarded that young woman, as a thing? And what had she ever said to her except ‘I hear you’re getting married, Angela’ or ‘How pretty you look, Angela.’ Had anyone asked about her ideas, her hopes, her plans?

The judge correctly interprets how Angela might really feel and react to always being treated like a pretty little toy of sorts. The judge even admits to herself that she would have reacted violently to people treating her in the same manner that they treat Angela.

If I had been treated like that I’d have used dynamite, not fireworks; no, I would have just walked out and kept right on going.

It turns out that Angela is the bomber, and she did it as a way to intentionally harm and scar herself. Once her image isn't tied to her beauty, she is free to actually act as herself.