Why "le" here?

Why "le" here?
 

Here is the phrase:

Es de buena educación abrirle la puerta a las personas que llegan detras de tí.

First off, why the le there? Why not les? It seems strange to me, if anyone could explain. Also, what is the difference between ti and tí?

Thanks and Merry Christmas!

According to Butt and Benjamin (4th ed), section 11.16.3, titled "Le for redundant les", this is a strong habit in spontaneous language everywhere, especially for a human indirect object, but it is technically 'wrong', even though using "les" sounds odd to some native speakers. Even so, it should be avoided in formal writing: Butt & Benjamin state that often the best way to avoid it is to omit the 'redundant' pronoun altogether.

The Diccionario Panhispánico de Dudas agrees that this is not the right construction, but the use of this lack of agreement between "le" and the plural to which it's referring to is a general use. That is why it may sound weird for some speakers.

@wafflestomp: You are right to think the word should be "les".
As for your second question, "ti" doesn't bear an accent. Many people write "tí", I think, because there is "mí" (personal pronoun), but this last one is accented to make it different from "mi" (possessive adjective). "Ti" is not confused with any other word and it falls into the category of one-syllable words that must not be accented, like fe, fui, vio, vi...