Learn French: Pourquoi est-ce que tu es en retard ?

Adding est-ce que right after a question word like pourquoi (why) is a very common way to ask questions in French. It acts like a signal that a question is coming, and it allows you to keep the rest of the sentence in the normal subject-verb order (tu es).

Why is it en retard instead of just using the word tard?

When referring to a person or a vehicle being delayed, French uses the phrase être en retard (literally, to be "in delay"). You only use tard when talking about the time of day being late, like saying "It is late" (Il est tard).

Can I ask this without using est-ce que?

Absolutely! In everyday spoken French, you can just use the question word and keep the normal sentence order: Pourquoi tu es en retard ? You could also swap the subject and verb to sound more formal: Pourquoi es-tu en retard ?

Does que drop its 'e' before tu?

No, in written French, que only drops its 'e' and becomes qu' when the next word starts with a vowel or a silent 'h' (like qu'il or qu'elle). Because tu starts with a consonant, it stays as que tu.