It's mostly, but not entirely, a combination of biases working in a feedback loop. Because of the 737 MAX crashes, which were legitimately at least partially Boeing's fault, stories about incidents involving Boeing planes are more likely to get covered in the media and become popular on social media, essentially selection bias. Confirmation bias means both the media and readers are more likely to view it the new story as Boeing's fault whether or not that's accurate, and this first impression becomes solidified in peoples' minds, anchoring bias.
This is very important because aircraft accident investigations take time and few aviation incidents generate enough attention that there is much if any follow up once the full cause is known. This first impression is what people remember the next time there is a Boeing story, further strengthening the confirmation bias. It's a self reinforcing feedback loop.
Which is not to say that Boeing doesn't deserve some bad press. The MAX story, the door plug blowout, and some others are legitimate evidence of issues at Boeing. But the bias feedback loop ensures that every Boeing story is treated that way, significantly distorting the reality of the issue.
