Why does the second person (tu) lose the 's' in the imperative? Tu t'en vas? / Va-t'en!

Bonjour Merle !

This is the rule applying to all -ER verbs in L'Impératif in French:
in the "tu" form, they lose the -s, unless they're followed by "- y" or "- en".
"Aller" though irregular -ER, still abides by this rule.
Have a look at the related lesson:
Conjugate regular verbs in the imperative mood in French (L'Impératif)

I hope that's helpful!
À bientôt !