thats because by default Page Peel will peel off the outgoing clip …. you need to tell the transition to act in reverse:
add the transition to the start of the clip
select it and press enter to load it into the viewe
directly to th eright of th esliders that control the start and end percentage of completion for the transtion, you will see a small rectangukar gray button containing a transition icon and a small right pointing arrow within
toggle that button and you will see the arrow changes to point left, that now indicates that the transition will work in reverse
see if that works now