I'm not certain but if I were to make a guess, I'd say the "Eclipse Cannon" the kingdom wanted to use was a lie and the gate opened to the wrong time allowing the dragons to pass through to ours.

[She's given this a fair amount of thought.]

They must have shut the gate somehow, we were told ten thousand dragons would appear but there are only seven and yet the odds are still against us...

[She'd almost died and now Erza was trapped here in a bed, about to return to a completely different battle than the one she wanted to fight. Having to abandon her family in a time of crisis...]

I can't stand this, Natsu...