I ask because the first few times I shopped there, years ago, a security guy politely asked me to take my shopping bag out of the trolley and hang it on the hook near the handle.
I shop at Aldi loads and have never been asked to do that, but I know that they are v careful about people having stuff in their trolley that they shouldn't (saw a deputy manager doing a practice with a lass on the tills). I expect they get a lot of thieving with people putting just one of their multi-buys on the belt - not that I am saying that is what you were doing.
They were probably running a sweepstake on how many gullible people would actually do it and not even question why. If they ask you next week to stand on one leg, I would shop elsewhere! ;-)
Jeze, I shopped there lots of times and was never asked to put my bag on the belt, that was the first time, could be they suspected me at the time and that was their way of sussing me out!