I got the letter today - well,two of them in fact, one from when I tried the CLI button and the other from my call to Credit Solutions.The reasons given were the same in both cases:
serious delinquency and public record or collection filed (this likely refers to my 2011 homeowners' association fee civil judgment; it's irksome that that still dogs me nearly six years after it was entered and three and a half years after it was satisfied);
number of accounts with delinquency (the HOA fees and my student loan; though the latter account hasn't been delinquent for at least three years now);
too few accounts currently paid as agreed (in other words, too few accounts, period, since I'm paying all the accounts I currently have as agreed; the problem is that TransUnion apparently won't record my Penfed car loan until I start paying on April 16th, and I haven't even started using my Discover card yet);
too many accounts with balances (OK, to me this last reason is just ridiculous. High balances I could understand, but just "balances"?)
Also, the TU score reported on the account website isn't even correct; it showed as 630 as of 3/6 online, but in the second letter it says 627 as of 3/27.
Pfffffft. 
Nothing for it but to garden for the next few months.