This British poll taken by an independent group found the likely reason Trump's poll numbers are declining recently

In early June, the politically independent public policy research organization, Democracy Institute,  President Trump was ahead of former Vice President Joe Biden by 48 percent to 47 percent with 5 percent undecided. It was a poll of 1500 likely voters, cell and landline, that was done by an automated system randomizing the questions. They found equal numbers of Republicans and Democrats..As of April 2020, Gallup polling found that 31% of Americans identified as Democrats, 30% identified as Republican, and 36% as Independent. Of course the numbers could have changed in the last three weeks. The polling has. RCP has Biden with a +9.4 lead now.  Most of the news media and many pundits claim Trump's support is collapsing due to the rioting and looting making Americans feel less secure and driving them to Bide. That seems contra logical to me. Biden hasn't even spoken out against the rioters that I can find. Instead, he has supported protestors while mostly hiding in his basement. 

So, what is another possible explanation? The British poll by the Democracy Institute has a strong candidate.

However, only 37 percent of Trump voters would want their friends and family to know how they had voted while 74 percent of Biden supporters are comfortable with it being known, suggesting there is a “silent vote” for the President.

President Trump's supporters have been driven underground by the relentless attacks from the press, liberals and Democrats, but I repeat myself. The Democracy institute poll didn't force those polled to talk to an actual human.This NBC/PBS/Marist Poll did and found Biden up by 8 points. The most recent (2 days ago) Hill-HarrisX poll, which is conducted online, found Biden only up by 4. The NY Times/Siena poll that found Biden up by 14 also uses live interviewers.

The data would seem to suggest the hidden Trump supporter part of the electorate has increased.