I don't believe that there's a need to speculate or assert that it was the gift cards when the facts outlined by OP show that: first shutdown happened on the banking side which has no connection to the gift card transactions on the cc side; the cc's were shutdown b/c OP was shutdown by the banking side (the letter states this much instead of saying something along the lines of unsatisfactory relationship); OP mentioned that there was a pretty consistent pattern of behavior in terms of buying giftcards for 2-3 years without any issues; and, we also know that Chase is very risk adverse on the compliance side when it comes to banking transactions. We also now know that in addition to the new/high amounts of ACH transfers, OP also mentioned this:

Now, I'm not passing judgement on OP for any of this, but one can see how this new pattern of banking transactions probably triggered a security review with the outcome being that Chase decided to just terminate the banking relationship (it wasn't just ACH transfers). This is not a liking issue, but compliance issues with ML laws on the banking side, which are no joke as the bank incurs liability if it fails to maintain programs to prevent against ML. I'm not saying that this is what's happening here, but Chase probably has a prevention program in place with tolerance levels set very low in order to take no chances.

Buying gift cards, while they may create profitability issues because of the increased rewards, does not create any compliance issues/security concerns for the banking side. The first domino fell on the banking side, and the cc side just followed suit (they probably have an automatic notification system that lets the cc side know when someone is shut down by the banking side).