Google has confirmed in a Google AdWords Help forum thread that there is a bug with some advertisers not being able to see why some of their ads are not approved. In short, when an ad is disapproved, it doesn't show why, normally it does tell the advertiser why.
Here is a typical ad disapproval bubble that shows why the ad was disapproved:
Again, some advertisers are not seeing anything but a disapproval notice but with no reason on why the ad was disapproved.
Cassie from Google said in the thread, "I've just confirmed that some other advertisers are experiencing this as well." "The engineering team is currently working on a fix," she added.
She recommends the advertiser calls AdWords support to get help on why the ad was disapproved until this bug is fixed.
Forum discussion at Google AdWords Help.