Long text messages are auto broken as they are being composed - why?

As I'm typing a text message there is a little counter at the bottom of the composition box that starts showing the length of the message when I hit 130 out of 160 characters.  Once I hit 160 characters the message indicates I have moved to two messages and continues to track the character count.  The message looks to be intact as typed, except for the little counter indicating I now have multiple messages.  Didn't realize what it was doing until I sent my brother a text and said his phone was going crazy dinging him with notices of new texts - all of which were the result of my one long text. I don't seem to have the problem on the receiving end, can't go back and check as I delete the history soon after I'm done with the conversation.  Is this an AT&T "feature" or a Window phone 8.1 update 2 "feature" or a Nokia Lumina 635 "feature" - and what can be done about it (other than send shorter messages) so I don't multi-ding future recipients?