Why is the request URL / HTTP_REFERRER in Django error from an external domain?

TL;DR

Was getting a Django error because I wasn't properly redirecting authenticated users making GET requests to my login endpoint (fixed this problem); however, I'd like to understand why the Request URL / HTTP_REFERRER in the error message was an external domain.

Background

I've been getting a few Django server errors per month in my production web app that list sendgrid.net as the Request URL (as well as the HTTP_REFERRER). I realized that some of my transactional email templates were turning links into strange sendgrid URLs for clicktracking, so I added clicktracking="off" to all the anchor elements in my templates to avoid this. However, the errors have persisted.

Current Problem

Today I tried navigating to https://example.com/login/ when I was already logged in, expecting it to redirect me to my user profile page but instead it triggered a 500 error. When I looked at the error readout that was automatically emailed to me from my Django app, the Request URL / HTTP_REFERRER was not https://example.com/login/ but rather a subdomain of sendgrid.net (see below).

The error was being triggered on my view called CustomLoginView:

Exception Type: NoReverseMatch at /login/
Exception Value: Reverse for 'member-detail' with no arguments not found. 1 pattern(s) tried: ['member/(?P<slug>[-a-zA-Z0-9_]+)/\\Z']
Raised during: home.views.CustomLoginView
Request information:
USER: Doe, John: john.doe@example.com

The Sendgrid Request URL

The URL is structured like this: https://u<8-digit ID number>.ct.sendgrid.net/ls/click?upn=<extremely long string of characters>.

views.py

Note that the view did not have redirect_authenticated_user = True before (which was causing the error). I added that line to fix the error.

from django.contrib.auth.views import LoginView

class CustomLoginView(LoginView):
    """
    need custom login so I can set session timezone upon login
    """
    redirect_authenticated_user = True  # This is what I added to fix 500 server error

    def form_valid(self, form):
        """Security check complete. Log the user in."""
        auth_login(self.request, form.get_user())
        # now set session timezone according to user's preference
        tzname = self.request.user.tz_preference.key
        self.request.session["django_timezone"] = tzname
        # and activate it (after this view, middleware will activate it)
        timezone.activate(zoneinfo.ZoneInfo(tzname))
        return HttpResponseRedirect(self.get_success_url())

What is the cause?

The way I see it, there are two problems here:

My CustomLoginView is not properly redirecting already-logged-in users to their dashboard when a GET request is made to CustomLoginView.
Somehow, Sendgrid is involved in a way I cannot begin to understand. How can this subdomain of Sendgrid be hijacking a request to my domain?

I fixed problem number one by setting redirect_authenticated_user = True on my CustomLoginView, but I can't figure out why the Request URL / HTTP_REFERRER were from this Sendgrid subdomain.