Because this is chance encounter with 〜ので

Grammar Explanation: Objective reason with 〜ので

When Cocoa picks up another student’s dropped handkerchief, she uses the opportunity to show off her devilish nature to Chiya they discussed earlier.

ココア:: 「これも何(なに)かの縁(えん)なので今度(こんど)お昼(ひる)一緒(いっしょ)にどうですか？」; "How about we have lunch together next time, since this, too, is a meeting of chance?"