This is due to the donor character of the polar group. Polar groups increase the HOMO energy of the σ-system in the transition state. This causes the transition state to be in a much more favorable position when the polar group is not interacting in a 1,3 allylic strain.
The occurrence is attributed to the polar group's ability to donate. The polar group boosts the HOMO energy within the σ-system during the transition state. Consequently, the transition state becomes significantly more favorable without being hindered by 1,3 allylic strain.