Well,it could be caused by a number of things - a signature in the text, special formatting, even a text that's simply too long...you should get in touch with Verizon and see if they can't clear up up.
i'm verizon tho. :/ its every time i send a long-ish message, if it did that when i was sending a message to non verizon phones, then i might understand,but it's to verizon phones.
Get a hold of your provider and ask them what's with your phone. --If you have droped it or something,that could cause some issues. that happened to my phone once.
Who is your provider? I know Virgin Mobile was having an issue with that a few weeks back, but I would figure it fixed by now.