The behavior that you describe cannot happen as a result of a "regular" postback. Perhaps you use AJAX and partial page update, and while the part with the button gets updated, the tooltip probably does not and stays "hooked" to the old/disposed version of the button.
By the way, the ShowEvent property must
be set ShowEvent = "OnClick"
It would be best if you open a support ticket, and send us a working project that demonstrates the problem. Right now, there is information missing.
the Telerik team