Do you have a save prior to the attack that you can reproduce this from? If so, it may be helpful.

I have been able to also find this.

This is related to the fact, that units do not move as an army but move individually if you select them like shown in the screenshot below.

In the case that you select units in this way and attack someone the units fight one after the other with the enemy. This happened to me also when attacking an enemy selecting some units in this way (i did not want to include my pioneer in the attack) and 7 of my units were then killed one by one by a seemingly weaker enemy.

The attack works just fine if you do NOT mark the units as shown in the screenshot.

Here is the corresponding savegame to reproduce the issue.

http://dl.dropbox.com/u/9038574/Attack.EleSav

Conclusion: The army management needs some serious polish. There are several suggestions in the beta and Ideas forum on this matter.

This is potentially a much bigger issue when attacking out of the city garrison because that's the only way to select units.