I recently obtained DCU membership and made my $10 donation, and $5 deposit into savings.
I did a little searching in the forums to see if it would code as a cash advance, but didn't see anything recent. Now I see I was using the wrong search terms because "DCU Funding" comes back with a warning from 2014: Funding a DCU account will code as a cash advance. I don't see anything more recent than that, so I'm posting this as a warning to others.
I just took a look at my Capital One account online, and I see two transactions ($5 and $10) and two $10 cash advance fees as well. 
I'm sorry this happened to you. 
FWIW, there's a great DoC article with a list of many banks/CUs and how their funding codes: http://www.doctorofcredit.com/does-funding-a-bank-account-with-a-credit-card-count-as-a-purchase-or-...
There's no entry in the article for DCU, but down in the comments there are a couple of people who had the same experience as you, so you're definitely not alone. 
