# Carbocation rearrangement in alkene addition reaction

This is a question from my textbook:

Determine a step-wise $S_N1$ mechanism for the overall reaction:

The solution shows the following carbocation rearrangement after Bromine leaves in the first step:

Why does the carbocation rearrange like this, going from a tertiary allylic carbocation to a secondary allylic carbocation? I thought tertiary carbocations are more stable and thus favored to be the major product? I considered Zaitsev's Rule in which the more substituted alkene is the major product, but I thought that only applies in elimination reactions.

Any clarification is appreciated!