What does this mean?

This has been popping up lately and I have no idea what it is. 

That means your gate been attacked by player who you attacking. That happens if u missed some units from attacking players they starts to attack your gate and if they destroy your gate before you you will lose that attack.

So they are attacking me at the same time?

If you miss many troops, they can even destroy your gate and that means a defeat. 

 

Sometimes that pops up when i first start down the path and before i encounter any enemy troops. Must be from my troops trying to get thru the portal all at once. Swords and other equipment smacking the portal as they go thru