Broken TPMS sensors

A few months ago I bought new wheels for my 2007 Touring model at Discount Tire. They moved the TMPS sensors to the new wheels, mounted the tires and off I went. About 3 weeks ago the warning light started blinking and then stayed on. I grabbed my owners manual and read that if it blinked for 1 minute and then stayed on it meant that there was a problem with the system. Sure enough, my trusty second hand confirmed that it would blink for 1 minute and then it stayed on steady. I took it into the dealer on Saturday and they said that the computer showed that it was only getting two signals. I had them fix it so they got me 2 sensors for $107 a piece and charged me $206 to install them. I found out that their shop rate is $101... unbelievable. Anyway they found that the front two sensors had broken. I left after paying the bill only to get most of the way home, around 20 miles away, just to have the light come on again. Now my service advisor told me that they had checked all 4 sensors. They're trying to tell me that Discount Tire must have installed them incorrectly and that's why they literly broke apart. My question is, could this be true. I watched them put one in the new wheel and it didn't look that difficult. I read something in another post about the angle of the sensor. If the angles didn't match could that break it. What bothers me the most is that the technician supposedly checked the other two and thought they looked okay. If anyone has an opinion on this I'd like to hear it. I like having the system working, it's one less thing to worry about when I'm driving.

Thanks and sorry for the long post

Dave