Check your signal strength. If it's below -90dBm you're in a marginal signal area and 1x might be the only way it can connect to the tower. Another possibility is weather or maintenance being done (or in desperate need of being done) on your tower.
My D/X has done this from day one of 15. Called Verizon speaking with level 2, but no explanation. Nearby cell tower supposedly is fine. Emailed Mot but no reply. No reason to the 3X to 1X and back to 3X. One rep at Verizon said the radio in the D/X was not as good as the Blackberry and his opinion was that may be the reason. Opinion. I see I am not the only lucky person experiencing this as it is on nearly D/X forum.
Depending on signal demand, Verizon will throttle phones back to try to prevent overloading the tower... Feel free to flame at this point, I just put on an asbestos suit.