Are you talking about an app download or a system update? If the system update, being rooted if its coming from Verizon is problematical.
If an app download, could be any number of things including a poor connection at the time. If this is the case, try doing a *228 (#2) which will update your roaming capabilities (ie: connect you to the nearest and strongest tower). This might help.
Also, before attempting the download, try a battery pull (battery out 1 minute minimum, back, turn on). Often takes care of misc bugs. If you have been downloading a LOT of apps in a short time, the battery pull should be done along the way and after you are done. This allows the apps to "settle in."