It' a grammar rule: in French you have to insert a hyphen between the verb and the subject pronoun when they are inverted, whatever the reason for the inversion. That means we need a hyphen:
- When asking a question: Où es-tu ?
- In incidental clauses: Je suis dans ma chambre, répondit-elle.
- And whenever a word or a phrase requires the inversion.
Aussi placed at the beginning of a sentence is a coordinator and marks a consequence or an explanation. In this case, the verb and subject of the sentence it introduces can be reversed.
In the case of aussi the inversion is not compulsory (Aussi un groupe de matelots se livrait ... would be possible). In some cases the inversion is compulsory, it is the case with peut-être (Peut-être est-elle dans sa chambre), encore (Encore faudrait-il qu'elle soit arrivée), sans doute, probablement, and a few more.
Note that in the imperative there's no subject, just a verb (Parle !). I expect you meant the imperative of reflexive verbs, as in Lave-toi les mains ! where toi is the reflexive pronoun.