Disregard SVP in the sign above. The imperative requires assieds-toi/asseois-toi! or asseyez-vous! But here, I ask about using infinitive as imperative, not the imperative itself.
To add the excellent answer Toto gave, the reason that
vous precedes the verb is that this is a verbe pronominal. Notice that on the conjugation of the verb that the verb is preceeded by the pronoun in all the modes except the imperative, where it follows the verb (typical of verbes pronominaux).
This raises an interesting question: why does the imperative "break the rule"? While I couldn't trace the origins of the actual rule, it would seem that in French we simply don't like starting the sentence with a direct object pronoun. In fact, notice that the pronoun reverts to before the verb as soon as we negate the clause :
Ne vous asseyez pas là !
I suspect a lot of the uneasiness people are feeling at the wording in this sign isn't because of its syntax, but of the person chosen: infinitives of injunction typically are fairly impersonal and as such use the third person.
Switch the sign to "S'asseoir ici" and the awkwardness disappears.
As for the core of your question, object pronoun position in French is fixed (unlike say Spanish): they're always are placed before the verb unless that verb is a positive imperative (and sometimes a negative imperative too, in everyday speech)
Since the verb in the sign isn't in its imperative form, there's nothing surprising to the placement of its pronoun.
That the verb is an injunction is irrelevant: it is not verbal semantics that decide pronoun-verb or verb-pronoun order, but the formal characteristics of the verb
It's due to pronomial or reflexive verbs. The sign is missing something grammatical for the word order. However wrong it may be in syntax, @jlliagre added in a comment a potential reason for the error:
"An ellipsis of Veuillez, possibly due to lack of space, might also explain the formulation:
Veuillez s'il vous plait vous asseoir ici."
That would make it grammatical. Wanting to keep SVP took up a lot of space. One word could make the difference but since it's a sign, paucity of space could lead to poverty of syntax. The register also differs between "asseyez-vous"/"assoyez-vous" and "veuillez vous assoir."
Since it's a pronominal verb, the reflexive pronoun is in effect a part of the infinitive.