Evacuations take time. You need to organise an order in which people go and prioritise resources like transports (which may be ships, lifeboats, or horses). Leaders will typically either go last, marshalling people ahead of them and keeping an eye on proceedings, or first, leading the evacuees to a safer area and avoiding any incidental dangers along the way (think Théoden leading the Rohirrim to Helm's Deep).

PCs are likely to be, or naturally adopt the role of, leaders if caught in an evacuation. Some of them may want to go first, some way want to stay behind. And since danger is imminent, it's easy for groups to get separated.

Sounds like a perfect way to split the party!

Transcript

Rebel Trooper: Cold out.
Sergeant Major Trey Callum: Yah, you betcha.
Rebel Trooper 2: {faintly, in background} Why in tarnation did Major Derlin order us out here when everyone else is evacuating?
Rebel Trooper 3: {faintly, in background} Do you want to build a snowman?
Rebel Trooper 4: {faintly, in background} Sure. The cold never bothered me anyway.
Leia: Prepare for aerial assault. We won't have time to evacuate everyone before they start bombing.
R2-D2: I've spreadsheeted an evacuation schedule. General, as ranking officer, you'll be in the very first transport. You'd better hurry, it's about to leave.
General Rieekan: Droids don't give orders. I give orders, and I'm staying.
R2-D2: But...
General Rieekan: My word's final. Are you deaf as well?
C-3PO: Well played, Artoo.
R2-D2: Shhh!