On top of all that, the tribes of Judah and Issachar are adjacent to each other (i.e. Iudas Issakhar in LXX) in Deuteronomy 27:12, and the following curse occurs a few verses later: "Cursed is the man who accepts a bribe to kill an innocent person" (v. 25).
Perhaps the Number 7 passage that deals with the inaugural sacrifices at the Temple are relevant as well. Notice the first two to bring sacrifices and gifts to the alter were tribes of Judah and Issachar.
10 And the rulers brought for the dedication of the altar, in the day in which he anointed it, and the rulers brought their gifts before the altar.
11 And the Lord said to Moses, One chief each day, they shall offer their gifts a chief each day for the dedication of the altar.
12 And he that offered his gift on the first day, was Naasson the son of Aminadab, prince of the tribe of Judah (Judas)....And he brought his gift, one silver charger of a hundred and thirty shekels was its weight, one silver bowl, of seventy shekels according to the holy shekel; both full of fine flour kneaded with oil for a meat-offering....15 One calf of the herd, one ram, one he-lamb of a year old for a whole-burnt-offering;
16 and one kid of the goats for a sin-offering..18 On the second day Nathanael son of Sogar, the prince of the tribe of Issachar, brought .
19 And he brought his gift, one silver charger, its weight a hundred and thirty shekels, one silver bowl of seventy shekels according to the holy shekel; both full of fine flour kneaded with oil for a meat-offering.
The close proximity to the words "silver" and 'thirty shekels might have contributed to this usage.