I don't use the Toolbar control myself, so this is purely speculation but:

I can imagine that clicking the toolbar has been designed to raise Validate events because in many ways toolbars can act like buttons. For example, let's say you have a Save button on your toolbar - you'd likely want validate events to fire before you performed your save operation.

How about this test: if you set Cancel = True in the TextBox validate event, does the Toolbar button clicked event still fire?