The phone selects a RAT (radio access technology) and frequency for each SIM slot.

After selecting, each SIM slot is subject to inter freq / inter RAT reselection / handover.

Both are controlled by messages received from the tower (e.g. on 4GLTE, for reselection, System Information messages), though there is an additional constraint: what's supported by/enabled in the phone.

Perhaps one SIM slot was in the connected state and the other was in the idle state at one point. So the reselection logic applied for one and the handover logic applied for the other. There is for example a problem called ping pong handover. Once a phone is switched to a different frequency or RAT, the tower may have the phone be sort of stuck in the new frequency, until the conditions of the previous RAT or frequency improve substantially, in order to prevent the phone being like a ping pong ball between the two. This frees resources that would otherwise be spent on repeated handover-related messages.

Each frequency has its own signal strength (free space path loss, transmit power, one frequency might be on one tower and another might be on another, etc).