It is the plural of respect. Think of Shalom Aleichem, which you say to an individual.
I will (now) add that this is a common feature of many languages. While not true for Biblical Hebrew (though there is the plural of mastery), looking at the entry on T-V distinction on Wikipedia:
Yiddish makes use of the second person plural form as the polite form for both singular and plural. In the second person plural form איר (ir), there is therefore no distinction between formal and informal forms. There is a dialectal pronoun עץ (ets) strictly for informal second-person plural form, but this pronoun is rarely used today and is only found in some dialects of Poland and neighboring regions.
Given that old German dialects were the main influence on the development of the Yiddish language, this form may be recognized with older polite forms of the German language.
So it might well be an influence of Yiddish, which indeed could exist more in more religious circles.
It could also be influence from Arabic:
Modern Standard Arabic uses the majestic plural form of the second person (أنتم antum) in respectful address. It is restricted to highly formal contexts, generally relating to politics and government. However, several varieties of Arabic have a clearer T–V distinction. The most developed is in Egyptian Arabic, which uses حضرتك ḥaḍritak (literally, "Your Grace"), سعادتك sa‘adtak and سيادتك siyadtak (literally, "Your Lordship") as the "V" terms, depending on context, while أنت inta is the "T" term. Ḥaḍritak is the most usual "V" term, with sa‘adtak and siyadtak being reserved for situations where the addressee is of very high social standing (e.g. a high-ranking government official or a powerful businessman). Finally, the "V" term is used only with social superiors (including elders); unfamiliar people perceived to be of similar or lower social standing to the speaker are addressed with the T term inta.
Regarding Hebrew, they have this:
In modern Hebrew, there is a T–V distinction used in a set of very formal occasions, for example, a lawyer addressing a judge, or when speaking to rabbis. The second person singular אַתָּה (ʔaˈta, masculine) or אַתְּ (ʔat, feminine) are the usual form of address in all other situations, e.g. when addressing ministers or members of the Knesset.
The formal form of address when speaking to a person of higher authority is the third person singular using the person's title without the use of the pronoun. Thus, a rabbi could be asked: ?כְּבוֹד הָרַב יִרְצֶה לֶאֱכֹל (kəˈvod haˈʁav yiʁˈtse leʔeˈχol', "would the honorable rabbi like to eat?") or a judge told: כְּבוֹדוֹ דָּן בְּבַקָּשָׁתִי (kəvoˈdo dan bəvakaʃaˈti, "his honour is considering my request").
Other persons of authority are normally addressed by their title only, rather than by name, using the second person singular. For example, officers and commanders in the army are addressed as הַמְּפַקֵּד (haməfaˈked, "the commander") by troops.
In non-Hebrew-speaking Jewish culture, the second-person form of address is similarly avoided in cases of higher authority (e.g., a student in a yeshiva would be far more likely to say in a classroom discussion "yesterday the Rav told us..." than "yesterday you told us..."). However, this usage is limited to more conservative (i.e. Orthodox) circles.