Depending on the age of the starter, could be old age.
However, the condition you're describing is known as heat soak. The starter gets hot, expands, and stays stuck until its cooled some.
A couple of fixes are to get a starter heat shield and install around starter, get a solenoid kit from Painless or Summit Racing, or it just may be the battery cables are too small of wire gauge. All of these can be purchased from Summit Racing.
Typically, battery cables should be no smaller than 4 AWG, 2 AWG would be alot better. And, of course, a clean connection at both ends of both cables.