Does it have something to do with MIMO?
Yes, it could do. MIMO (multiple-input multiple-output) can use different polarizations to transmit two data streams in the same space. Lowest link loss occurs when the transmitting and receiving antennas are aligned to the same polarization. At 90° the signal strength is reduced (theoretically to nothing, but in practice by a bit less than 20 dB) so the two signals can easily be distinguished even when on the same frequency.
Another reason for using two antennas is simply to relax the need for a specific orientation. This is commonly done in portable devices and other things that cannot maintain a fixed antenna orientation (eg. drones). In this case the same signal is sent through both antennas, and the receiver uses whichever signal is best.