If this consistently happens can you provide any evidence in any election other than the 2016 presidential election and the 2014 elections? Those were both years where there is pretty strong evidence that there was a late shift to the Republicans, but if this is something that happens every cycle then there should be evidence for it every cycle, or at the very least more than two.
It certainly did not happen in 2008, where McCain was actually ahead in September before the polls shifted to a big Obama win. (And on average the polls performed very well).
Source.
It certainly did not happen in 2012 where polls continuously oscillated from being about tied to having a slight Obama lead. (Polls actually overestimated Republicans at the end and never once did the RCP average after April did the RCP average reach what Obama actually got. How could the polls have been oversampling Democrats the whole time if they never even averaged Obama's actual result?).
Source.
I cannot find any Senate election where it happened in 2010, 2012, 2016, or 2018.
Source. Source. Source. Source.
If this were something legitimate that happens, then it should show up in the data. It's not something you can have an opinion on. Either the pollsters are consistently overestimating Democrats in the summer and then showing tightening at the end or they aren't. And from all the data I've seen, they aren't.
That's before even getting to all the horrible incentives polling companies would have for doing this. They get business based on how well they poll elections, and the public polls these firms do for media companies are not a huge part of their business, but are by far the most public. Intentionally manipulating their data in a way that would be obvious to someone on an internet forum would be such an insanely bad business strategy it would make no sense. It also doesn't make any sense from a strategic perspective if they secretly wanted Democrats to win. Oversampling Democrats in the summer gets Democrats nothing. Maybe at the end you would want to do that to depress the other side's turnout, but according to your scenario the pollsters would already have switched back to being accurate. I strongly, strongly disagree with your theory here.