First of all, not all UX flights operate from the A gates - some use a few of the higher-numbered, even-numbered C gates.

Second, the shuttle from D to A only operates during 4 busy periods of the day (probably including your arrival time). But that shuttle usually only leaves every 8 minutes for a 4-5 minute ride. If you arrive at one of the low-numbered D gates (definitely D1-D7, possibly the higher ones too), you would be better off hiking into the connected C concourse and taking the C-to-A shuttle (leaves every 5 minutes for a 2 minute ride).

Finally, if you arrive 10-15 minutes early you should have no problem. If you arrive 10-15 minutes late (and the connecting flight is leaving ontime), hope you have a backup plan.