I came across this blog post from Marlon:
While running around in the WPF forums, I found this very interesting post… It seems that the WPF TextBox can be dangerous 
- if you do not use it carefully
Basically the TextBox by default stores all text changes in an Undo Stack… This implies that every time that you change the Text of the TextBox more memory is consumed by the TextBox.
In order to fix this you have to set the UndoLimit property to 0. This would disable the Undo feature for the TextBox and thus NO MORE MEMORY CONSUMPTION …
The MSDN documentation on the UndoLimit property:
Setting UndoLimit clears the undo queue. When UndoLimit is set to –1, the undo queue is limited only by the memory that is available. When UndoLimit is set to 0, undo is disabled on the TextBoxBase.
UndoLimit is introduced in the .NET Framework version 3.5. For more information, see .NET Framework 3.5 Architecture.