Static Electricity Maybe?
I am not a fire investigator at all. However I spent 6 years working in the wireless industry. This sounds to me like somehow the phone is creating entirely way to much static electricity. I would be interested to know what model phones they were by nextel It could be defective parts in certain models causing it. if you say regular cell phones do not cause this then the only other thing that could cause it is the two way radio portion of the nextel phone. Which lead me to wonder if other service providers two ways could cause it as well. Such as Verizons Push to talk. (they essentially are identical only different service providers.) When I left the wireless industry the only ones making direct connect phones were Motorola. I also would look into is were the phones in any way altered by the owner outside of normal manufacture guidelines and did they have some sort of signal boost device on them. That could create the problem. There have been reported cases were static electricity from phones have caused car fires when using them while fueling up at the pump. This could be a relative similar thing.
It trips on multiple manufactures
I had a breakfast bar installed in my kitchen and the GFI outlet which is right under the counter of the breakfast will trip every time a cellular call comes in or a text message. When using the Nextel 2way feature it does not trip the GFI. The phone must be within a few inches of the receptical to trip the GFI. We changed out the receptical with another one of the same model, same result. We then tried a different model and still the same result.