Update - 1:30 EST
As of 1:30, Switch firmware update 12.0.3 is live and able to be downloaded (thanks VGC!).
The day prior to the update's launch, dataminer OatmealDome posted what the update is meant to change, namely, how the system establishes secure connections and some changes to the OS kernel.
So it seems like there's nothing to see here regarding possible connections to a Switch Pro announcement.
Nintendo aren't usually that adventurous when it comes to updating console firmware. Usually offering minor stability updates, they released Update 12.0.3 for the Switch last night, which promised "general system stability improvements to enhance the user's experience."
However, in what appears to be a first for Nintendo, this update has suddenly been suspended only hours later, without official explanation. However, it's worth noting that Switch consoles download these updates in the background, meaning it may still be available if your Switch has been left in sleep mode.
Nintendo Suddenly Pull Switch Firmware Update 12.0.3
While some have speculated there's information about the rumoured Switch Pro, others believe it's caused performance issues. Outside of the promised stability update, Nintendo dataminer OatmealDome previously shared further details about what's within, though this didn't feature anything particularly major.
Until Nintendo release official comment though, we can only speculate as to why. We've emailed Nintendo to ask for clarification but at this time, we've not received a response. We'll keep you updated if we learn any more as to why this has happened.