I notice that sometimes when you enter combat screen, you can already see where the enemies are.
This is good because you can position your troops accordingly.

However, sometimes you can't see the enemies until you start combat.

Why is that so?

Are there any factors that determine whether you see or don't see the enemies?

Based on initiative.
Who have highest - have first turn and deploys troops first.

Which is irritating, as it's often useful (especially with the scout, who gets initiative bonuses) to see the enemy's positioning first.

Well, to some is irritating, to others (like me) is enthralling since you don't have the luxury to know where to position your units, and so you are forced to adapt... one of the many brilliant characteristics of Eador...

Oh, no, I agree that it's a good idea. I just don't think that it determines whether or not you know very well, as a high initiative is obviously intended to be a good thing. The 'one side positions first, one side moves first' method tends to even up the benefits: good for when it's determined randomly, bad when one side is being 'rewarded' for a high stat.

In my opinion, it would be better if the person with the higher initiative went first in moving, AND got to set up second. Initiative is, after all, reactions and ability to respond to an issue...

You have most of your answer here already but just to add a bit to it there are skills that give Initiative for each of the classes. For Wizard classes it's Wand Mastery skill that gives extra shots with an ammo, better ranged damage AND +1 Initiative.

The Scouts skill, can't remember the name atm, gives the Initiative as well as Ranged Defens and Resistance boost.

Warriors is I believe Athletics which also gives better stamina recovery, increased speed (movement), and the Charge ability.

I think the commanders has something todo with his siege ability as well but have not really played the class so don't remember.