Signing onto Shoprite Wifi causes Google to lock my account

Signing onto Shoprite Wifi causes Google to lock my account

Shoprite is a chain of grocery stores in the Northeast. They now have a
special iOS app where you can scan your own groceries as you shop. When
you leave, you simply pay for what was already scanned and skip the
checkout line.
In order to use the app, you must be logged into the Shoprite WiFi
network. However, every time I do that, I get an immediate notification
from Google that says
Google Account xxxxxx Suspicious login detected. See google.com/blocked
The link goes to a webpage that says:
We blocked an application from signing in to your account
We recently denied a sign-in attempt from an application (like an email or
chat client) or a mobile device because we weren't sure it was really you.
To prevent unauthorized access to your account, Google notifies you via
email and/or SMS when we notice this type of suspicious sign-in attempt.
After that, my Gmail account is locked, and I have to request a text
message to unlock it.
This is really strange since my Shoprite account doesn't have anything to
do with my Google account. I have seen absolutely nothing on the Internet
about this, and neither Google or Shoprite seem able to tell me exactly
what is going on.
I have no idea why this is happening, and I thought maybe someone here can
explain it to me. There are two explanations I can think of:
Shoprite is attempting to access my Google account. They're a pretty big
public company, and I can't imagine them surreptitiously doing something
like this.
My iPhone is attempting to access my Gmail while on Shoprite's WiFi, but
why would that matter to Google? I've connected to other WiFi networks
around (Amtrak, Starbucks, Cablevision, etc.) and Google doesn't care
about those. Besides, the app that's accessing my Gmail account would be
my iPhone's Mail.app which has been setup to do just that.
Any ideas what is going on?