Where was the Sun at the time?
The sky is viewed through Rayleigh scattered light. Unpolarised light (or rather, the electric field in the electromagnetic radiation) from the Sun can be imagined to cause oscillations in the bound electrons of atoms and molecules in the atmosphere. These electrons then remit light as oscillating electric dipoles, with a polarisation in the direction of the oscillation. The greatest degree of polarisation occurs when the scattering angle is through 90 degrees (since one of the polarisation states of the incoming solar light is effectively "unseen"). The exact relationship between viewing angle and Sun angle will be a little more complicated since the scattered light arrives from different heights in the atmosphere.
Anyway, when the Sun is near zenith, the biggest polarisation effects will be seen near the horizon.
The details are readily found here.