It could be nothing - they could just be trying to sell me some crap I don't need but I have an unlimited data plan and I wonder why I've had so much trouble with my connection lately.
Well, I wouldn't put it past any ISP to throttle if they believe you're using more data than they think profitable. Unlimited usually means full speed until 250 GB and throttle for the rest of the month.
It is also possible that something is wrong with the cable connection and they're trying to correct the issue by contacting you so as to arrange a time to fix it etc...
Do you have an timewarner email account to see if they sent anything to you?