This is not because it's a question. You have this reflexive pronoun also in statements and commands:
Du legst dich hin.
Leg dich hin!
Note that the verb hinlegen is a separable verb, so in many tenses it becomes legen + hin, but this not the topic of your question.
The verb hinlegen is a transitive verb. Transitive verbs also exist in English. These are verbs that mandatorily require an object. A sentence with a transitive verbs is incomplete without an object.
Some examples from english grammar:
Also in German many verbs are transitive:
wrong: Der Schüler bedarf.
The student needs.
correct: Der Schüler bedarf eines kompetenten Lehrers. (Genitive object)
The student needs a competent teacher.
wrong: Dieses Haus gehört.
This house belongs.
correct: Dieses Haus gehört dem Bürgermeister. (Dative object)
This house belongs to the mayor.
wrong: Georg kennt.
correct: Georg kennt den Weg. (Accusative object)
Georg knows the way.
And in German hinlegen also belongs to the class of transitive verbs:
wrong: Simon legt hin.
correct: Simon legt die Blumen hin.
Most transitive verbs can also be used reflexive. This means, that the subject and the object are identical. To indicate this, you use a reflexive pronoun as object in German and also in English:
transitive: Maria wäscht ihr Auto.
Maria washes her car.
reflexive: Maria wäscht sich.
Maria washes herself.
In English you can omit this reflexive pronoun in many cases:
Maria washes herself. = Maria washes.
Markus lays down himself. = Markus lays down.
This is not possible in German. For some verbs it is possible, but then it changes the meaning:
- Maria wäscht sich.
This means that Maria is cleaning her body with water (and maybe soap or something similar).
- Maria wäscht.
This means, that Maria is washing something. The most likely meaning is, that she does the laundry. It definitely does NOT mean, that she is washing herself.
But "Markus legt hin" does mean nothing. This is just a wrong sentence. The word hinlegen is a verb that mandatorily requires an object that tells what Markus is laying down. And if he lays down himself, you have to say it in German.
Note, that there is another difference between German and English: English has no proper reflexive verbs. You can use transitive verbs in a reflexive manner, just as discussed above, but in English there is no verb that can't be used other than reflexive. In other words: When ever you find a verb that is used reflexive in English, you can use the same verb transitive too. This is true in English but false in German.
German has a lot of proper reflexive verbs, i.e. verbs, that always must be used reflexive. A usage of such a verb without a reflexive pronoun is wrong. Hinlegen does not belong to this class, because it can be used transitive too, but here are some German proper reflexive verbs:
Ich freue mich.
I am happy.
The correct translation of "I am happy" is "Ich bin fröhlich" or "Ich bin glücklich" which both have a similar meaning as "Ich freue mich." It is not possible to translate "Ich freue mich." into an English sentence that uses the same grammatical construction as the German sentence.
Barbara interessiert sich für Philosophie.
Barbara is interested in philosophy.
Same situation as before. The English sentence is in German "Barbara ist an Philosophie interessiert." Which means quite the same, but is an unusual construction in German. (Note that here the word interessiert is not a verb but an adjective.) The German sentence with the reflexive pronoun can't be translated into an English sentence that uses the same construction.
Other examples are
Bitte setzen Sie sich. - Please sit down.
Das Buch befindet sich in meiner Tasche. - The book is in my bag.
Herr Mayer beschwert sich über den Tee. - Mr. Mayer complains about the tea.