Mine did this, the previous owner had the linkage changed as it ceased (due to leaking washer jets) so he got another linkage and removed the existing motor and put the two together in rather a hurry, what he didnt do line the linkage up properly so everytime the motor was turned on it did a wipe then bounced back after it had completed its revolution ( know how annoying that can be!)
What you need to do is remove the linkage/motor from the car (scuttle off, wiper arms off etc) and if the centre bar is straight if not then it will need adjusting
After sorting mine i typically found this link which may offer some advice on how the linkage should look -
VWVortex.com - DIY - Removing the windshield cowl and fixing noisy / squeaky wipers on a MKIV car
This is for a mk5 golf but the linkages are the same so far as i can tell.