The obvious ones:

• She forgot.
• She's working.
• She's doing her thesis.
• She's out with her family.
• She's sick.
• She's already asleep.
• Crappy internet connection.

Ze weird ones (aka other reasons why she's always late):

• Her clock/watch is 2 days late. =)) Okayno. More like 3hrs late?
• She keeps forgetting something back in ze house.
• She had the date wrong.
• She got lost!
• She was asked to do last-minute errands.
• She got lazy. :P
• She woke up late.

Disclaimer: Not all of these are true. :P