Most people check other people out as a potential spouse all the time, but in most cases it stays at checking out and finding somebody else. Sometimes one gets stashed away as a nice plan-b. >Nice, but not really that in love/something isn't quite right(not sure, if there might be somebody better or the timing is bad right now or don't fuck at the factory)< One becomes unbeknown an option, that is being keep save at a locker in the back of the brain, as "there is still time". You might have done that yourself several times, with that cute/nice guy/gal that is out of your league, to close/far to/from your social circles or you haven't had the chance to interact toRightNowButTheChanceWillAppearI'mSureAndThenThereMightBeATinyChanceButMaybeNotYaddaYaddaYadda.
And then...BOOM a potential rival gets identified and the save option isn't save anymore. (In this case I believe the adventurer-chic and the new reception-trainee sniffed each others out.) Subconsciously an overdrive kicks in and the save option get interesting and rare and limited in time. The flirting suddenly intensifies from friendly jabs and everyday jokes (so no real flirting at all or barely noticeable and not really seriously meant) to more obvious smiling, slide attention seeking, when nearby by chance, and seeking out chances to be nearby by no chance (parties, smoking breaks, aso). That becomes noticeable, but it becomes noticable that more than one party is interested.
In most cases a new person one likes/befriended is the culprit and somebody one knows for a bit longer gets triggered. Those phases of popularity tend to blow over quickly as those people in question arrive pretty quickly at the conclusion, they won't lose that much of a chance in a perfect match anyways, or they would have gone for that person earlier anyway or they feel rejected, because their flirting leads to nowhere.
Feeling popular? Make a decision quickly and strike the iron while its hot. And If you can't decide, throw a coin and go with whatever you feel like. Those phases are fun bonus chances (If one can make a decision, that is.)