Using a transition to open screens helps users keep track of where they are and mimicks the native apps they're used to.
But on iOS Safari this leads to an issue. In Safari users can drag the edges of the screen to navigate to the previous and next page. This also works on iOS in native apps to close "pushed" screens (screens that animate in from the left). Keeping this behavior in place is likely desirable, but this can conflict with custom screen transitions.
If you pay close attention, you'll see the screen transitions from one side to the other twice. The first is Safari transitioning between the current and previous page neatly with the position of the finger of the user. The second is the transition from our app "catching up". We don't need both transiti