Units moving without orders?

[TotalWar]

Hi,
 

I just bought this game and like it a lot! Great stuff, as expected. However, I've encountered one problem and that is that units are moving even though I haven't given them a move command, told them to 'Hold' their position and in most cases haven't even ordered them to attack a specific enemy unit. But it happens in every battle that I scroll along my front line and I see troops moving around and I have no idea why (the majority does what I say, though). It's mostly infantry and rarely artillery. Only explanation I could come with is that units avoid standing in front of each other, but that doesn't make much sense since this works most of the time, both for me and the enemy.