For those curious about the technical details here, here's a dump of the iGrill Android manifest (it's not XML because Android APKs contain a binary manifest): https://pastebin.com/fwbJ9TjD

It looks like this is the culprit:

    E: activity (line=48)
        A: android:theme(0x01010000)=@0x7f110149
        A: android:name(0x01010003)="com.weber.igrill.pages.splash.SplashActivity" (Raw: "com.weber.igrill.pages.splash.SplashActivity")
        A: android:screenOrientation(0x0101001e)=(type 0x10)0x1
        E: intent-filter (line=52)
          E: action (line=53)
            A: android:name(0x01010003)="android.intent.action.VIEW" (Raw: "android.intent.action.VIEW")
          E: category (line=55)
            A: android:name(0x01010003)="android.intent.category.BROWSABLE" (Raw: "android.intent.category.BROWSABLE")
          E: data (line=57)
            A: android:scheme(0x01010027)="http" (Raw: "http")

This registers an intent filter for all HTTP (but not HTTPS) URLs. I would expect it to require a DEFAULT category though, not sure what happens when that's left out.