My money is on wifi not working right.
Wifi is a crapshoot in crowds that size, especially when you consider that > 90% of the fans in the stands have smartphones, all of which at least have wifi on and most of which probably have some setting that automatically connects them to open networks. At a minimum there's a bunch of RF noise from this alone.
It's worse if you consider the number of stadiums that install wifi -- I've never been to one where it worked well and in many it doesn't work at all. And stadiums themselves are often a clusterfuck of management, "operated" by the team in terms of cash revenue but managed by some stadium commission as a physical facility so that the local taxpayer can pick up the tab for annoying facility costs that aren't related to making the team owner richer.
It wouldn't surprise me at all if older stadiums retrofitted with wifi were done so on the local sports facility commission/taxpayer's dime and had all the usual corners cut as one might expect with such a project. The expectation (and effort) was probably decent coverage in luxury boxes, locker rooms and press areas. Fan seating areas get "covered" with a visible 2+ bar SSID, but nobody was willing to pay for RF engineering a workable solution for 70,000 people to actually use it.
So at best they're operating in RF soup with proper APs nearby, hoping that between signal proximity and operating on the 5 Ghz band they will get useful coverage. At worst they're working in RF soup off a crap solution.
Ideally, their software would be designed to be as network-independent as possible so that as much useful work as possible could be done without any network signal. But what do you bet it's a bunch of BS cloud based bullshit, dependent on appy Azure apps that Microsoft is hoping NFL teams and their corporate leaders will buy into even further.