Why would a cell phone suddenly consume huge amounts of data?

I'm putting on my elderly English-major hat to write this post requesting advice from some of the tech-savvy readers of TYWKIWDBI, because I don't know what to do.

Here's the story.  For the past 15-20 years I've used AT&T for landline and cellphone service with very few problems.  About two summers ago I was on a long road trip, sending photos home during the journey, and I received a test message "You used 75% of your plan data this bill cycle.  If you use all of it, your data will be slowed to 128kbps..."  I learned from that experience to bring the photos home or wait until I was near a motel wi-fi before sending them, and I've had no data usage problems since then.

Until April 15.  Taxes were done and filed (days earlier), and I was out in my garden clearing winter debris when the phone beeped withe the "75% data used..." message, which confused me because the phone was not in use.  And then, within probably 15 minutes I get a second message "Hi, it's AT&T.  You used all your plan data this bill cycle.  Your data usage will be slowed (max 128 kpbs)..."

That was a shocker.  I went into the house.  My wife was not using her phone (and we have no children or other phones on the plan).  The next evening I got online to track down and print out pages and pages of usage data.  The embed at top shows the start of the event, which continued like this...

So, basically 3 gigabytes of data consumed in about a ten-minute period.  

I phoned the AT&T help lines, which were singularly unhelpful.  The well-meaning staff probably have English as a third language and operate out of call centers with immense background noise or children playing, and were able to give me no useful answer as to what had happened (though they did try to upsell me some enhanced service).  

My next stop was an appointment at the Apple store to meet a tech at the genius bar.  He basically rebooted the phone, which improved its overall function, but he could not ascertain why this episode happened.  He went to the Settings to examine the Cellular Data which showed all the installed apps, but could not dissect out which one (if any) was hyperactive on the afternoon of April 15.  

I inquired about possible malware, which he considered "unlikely."  He thought perhaps one of my installed apps had gone rogue and called for data (???).  I have wondered whether someone hacked into my system - perhaps a neighbor who wanted to download a movie, or maybe someone driving down the street with some type of scanner looking for networks to hop onto for ten minutes to ?mine bitcoin or some other nefarious activity.  (We have a complex alphanumeric password that would take some degree of professional skills/equipment to hack).  

Things are not back to normal yet, with some residual problems seemingly affecting our gateway router and our access to premium streaming channels.  We have learned that TDS is in the process of laying fiber in our neighborhood and are considering switching to them from AT&T (who say they will have fiber but don't know when...).

So, an old English major would love to hear ideas about 1) why/how this happened, and 2) how to prevent it from happening again.  And if anyone has experience/comments re TDS Telecom fiber service, that would be a nice bonus.

Thanks in advance; perhaps your comments will help other readers of TYWKIWDBI with similar problems.