Most positive resists are based on Diazoquinones (DQ) or Diazonaphthoquinone (DNQ).
1.)  Start with a DQ molecule (R is a manufacturer variation).  The DQ molecule will not dissolve in a base developer solution (pH >7).
2.)  UV light breaks the nitrogen molecule off forming an unstable molecule.
3.)  To “stabilize” itself, one of the 6 carbon atoms in the ring pops out of the ring (leaving 5).
4.)  Once the solution is exposed to water (a developer/water mixture), an OH group attaches to the carbon atom, forming an acid.
5.)  The acid can then react and dissolve with the basic developer solution.