What shows up on the surface first
Within minutes of screeding, a thin sheen of water often starts to show on top of a fresh slab. That is bleed water. Cement and aggregate are heavier than the water mixed in with them, so as the mix settles under its own weight, the water works its way up and pools at the surface while the solids below it consolidate. Every properly placed pour does this to some degree. It is not evidence that the batch was off, it is the mix settling the way it is supposed to.
Why the crew waits on it before troweling
Finishing has to wait for that water to do its thing. Floating or power troweling a surface while bleed water is still sitting on top of it, or still rising up through it, seals a layer of water underneath a closed skin instead of letting it escape. Once it is trapped there, that top layer of concrete comes out weaker than the rest of the slab, and it tends to show up later as dusting, scaling, or a surface that flakes off in patches. The fix is patience, not a different technique. The crew watches for that sheen to disappear and the surface to firm up enough to bear a finisher's weight before the final passes start. That is part of why a pour finishes when it is ready and not when it is convenient.
What speeds it up or slows it down
Weather changes the timeline more than anything else. A hot, windy day pulls surface moisture away fast, sometimes fast enough that the surface dries before bleed water has finished rising through it, which opens the door to a different problem, fine cracking across the surface while the slab is still plastic. A cool, humid day slows evaporation down, so bleed water can sit longer and finishing gets pushed back later into the day. The mix itself plays a part too. A wetter mix generally bleeds more water than a stiffer one, which is one more reason the crew watching a pour matters as much as the concrete going into it.