When I sign in and scan a new build QR Code from https://build.phonegap.com/apps and try to install it on two different Android devices, the site redirects to Google or a Southwest Airlines promotional website without downloading or installing the app. Both Android devices have been used to previously download and install apps from the PhoneGap Build site as recently as late last week.
It sounds like a problem with your QR reader. I'm thinking you might have a free reader that is layering advertising on top of your QR code.
I had this problem with a QR reader on my iPhone...I scanned a QR from my book and saw advertising display at the bottom of the browser window. Initially, I thought it was the QR codes I created at qrstuff.com, but they explicitly avoid ads in their code (even in free QR codes).
It turned out to be the reader itself that was monetizing every QR it scanned by showing ads.
Are you able to manually download the apk to the devices? If so, try a different reader, and read through the reviews carefully for mention of advertising.
Thank You, Matt - duly noted! On our end, clearing out cache, cookies, etc. from both devices seems to have done the job.