This recent explosion of LTE outages on Verizon have had people asking if the 4G network is going down then why does my 4G phone not get 3G data? It all has to do with the way Verizon's 4G phones authenticate themselves with the network. I do not know everything about it, but from what I have gathered from many sources this is why it happens.
If you have bought a Verizon LTE phone, like the Galaxy Nexus, you know that activation is different than every other Verizon phone. You now have a SIM card. This is where the problems arise. Traditionally CDMA phones store the activation information inside the phone itself. LTE requires a SIM card and this posed a problem for Verizon. They would either have to activate to phone twice (SIM card and phone) or change their 3G network to work with SIM cards. They chose the latter. Verizon deployed a new variant of their 3G network called eHRPD. As far as I can tell this network allows a SIM to authenticate your 3G and 4G service together as well as smoothing out the hand-off between CDMA and LTE.
When this new authentication system goes down it causes major issues. It has been very buggy from the start. LTE phones are well known for getting stuck on 1x service. This is because this authentication from the SIM fails and the phone does not try to connect again for a certain period of time which may be a while. Toggling airplane mode fixes this because it makes the phone authenticate again and it normally goes through, but what if the authentication goes down completely on the network end.
When the network end of the authentication process ends it causes the 4G phones to fail to connect to the networks they are in range of. Hence, leaving them stuck with no data or 1x. If your phone was connected when the authentication system failure occurred then you will likely stay connected until the network tries to make you authenticate again or you change networks (move from 4G to 3G).
The 3G phones use the old authentication method and therefore are not kicked off their networks. The LTE and eHRPD networks are still up and running when this happens your phone just isn't allowed to connect to them. This may not be exactly what is happening, but it appears to be what it is to me. It would make sense why some phones are connected and some aren't.
Either way, Verizon really needs to get this fixed, but I doubt it will happen anytime soon as it seems that this system has some serious flaws that have yet to be addressed.
[I found this article on Android Police very informative on the use of SIMs on Verizon's network.]