He is overlimping with what he considers a speculative hand (an unsuited 1 gapper.. people do play these) Yet he expects his friend to bluff raise with ahigh frequency so he is planning on shoving if his friend raises.
Its caused by OP overlimoing rather than raising.
The same principle comes into play when you have a wide CO raiser, you are on the BTN with a medium hand that is ahead of CO opening range yet not good enough to call / raise a 3bet. The blinds are particulay 3bet happy and will 3bet very wide if the CO opens and the BTN calls. Therefore to prevent this you 3bet your weakish hand yourself to prevent the blinds from squeezing, yet would prefer to play the hand in a non-3bet pot.
you're really a stubborn one aren't you?
i bet if SB and BB high fived each other and openly discussed how they're going to split their winnings, you'd probably say "hey, that's not that suspicious, they're probably referring to winnings from the
blackjack they played earlier that day, and are both in a great mood and are high fiving for fun"
c'mon man. you really think a limp-happy 1/2 game has legitimate backraising spots and these two "friends" are really set out on creating really uncommon super aggressive dynamics between each other, rather than just colluding (in a manner that's pretty lol-tastic and easy to identify, as OP did) to take some easy cash from some bad live 1/2 players?
if you're just arguing that this doesn't 100% prove they're colluding, okay i guess... but it's just about as suspicious as it gets.
i think OP did a good job of identifying a collusion spot in this game.