Conspiracy theories aside, the best analysis I have seen regarding why the polls may overestimate Trump’s support (or the support for candidates Trump is backing) is due to sampling bias.
The pollsters do their best to capture a diverse, representative population for each survey round but it has become increasingly more difficult to get legitimate respondents, especially those in the younger age ranges, who by and large don’t answer their phones, are very skeptical of cold text messages and emails, and aren’t particularly motivated to participate in online surveys. And those are the respondents most likely to support Biden (or be ‘independent’).
Trump’s current support, in contrast, skews older and more eager to share their views, are much more likely to answer their phones or respond to cold text messages and emails, and are also more likely to be less truthful about their previous political behaviour and affiliations (not all polls undertake rigorous screening or post-response vetting).
Polling organisations do try to pre-select respondent pools to ensure a balanced sample, but when you are seeing declining response rates, and need to hit a magic number to be able to claim a 3% error margin, you do what you must to meet deadlines.
All of this may lead to polling that tends to exaggerate Trump’s support compared to Biden’s.
