The origin of the multi-band activities (outbursts/flares) of blazars is still a heavily debated topic. Shock and magnetic reconnection have long been considered as possible triggers for the multi-band activities. In this paper, we present an exploration of the origin of multi-band activities for a high-redshift (z =1.8385) FSRQ PKS 1502+106. Utilizing multi-band data from radio to $gamma$-ray and optical polarization observations, we investigate two dramatic activities in detail: a $gamma$-ray dominated outburst in 2015 and an optical dominated outburst in 2017. Our main results are as follows. (I) A fast $gamma$-ray flare with a flux-doubling time-scale as short as 1-hr in 2015 is discovered. Based on the variability time-scale, the physical parameters of the flaring region (e.g, minimum Doppler factor, emission region size, etc.) are constrained. At the peak of the flare, the $gamma$-ray spectrum hardens to $Gamma_{gamma} = 1.82pm0.04$ and exhibits an obvious curvature/break characteristic that is caused by the typical cooling break. Modelings of multi-band SEDs reveal a very hard electronic energy spectrum with the electronic spectral index of $1.07pm0.53$. This result suggests that this fast $gamma$-ray flare may be triggered by magnetic reconnection. (II) During the outburst in 2017, the optical polarization degree and optical fluxes show a very tight correlation. By analyzing Stokes parameters of polarization observations, our results show that this outburst could be triggered by a transverse shock with a compression ratio of $eta> 2.2$, and the magnetic field intensity of the shock emission region is about $0.032$ G.
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