I think a certain number of Trumps secret service want Trump to fail and fail in ways great and small. These people are likely to be embedded by Never Trumps or GOPe.
The SS just checks for criminal histories and threats. They do not check for political positions.
That is why most ex-POTUS have someone embedded with the SS to see the backgrounds and also to check for past comments.
Trump knows there is a target on his back. This could have been avoided easily. It is self-inflicted and gives his enemies ammunition unnecessarily.