You're probably running the two centers in parallel, which effectively cuts the impedance in half. This low impedance can cause your receiver to become too hot and damaged. Most receivers have protection built-in when it detects an issue, so that's why your receiver is shutting down. FYI, this could also ruin your speakers if the receiver doesn't shut down but instead clips the loud volumes, which could ruin the drivers.
One option is to wire the two centers in series, but this would probably decrease the output of each center channel and/or affect the sound negatively.
You should check the manual of your receiver and speakers. If your receiver can handle a 4 Ohm load, and your speakers are 8 Ohm each, the way you have it should be fine. If it's any other way (receiver can only handle 8 Ohm and/or the speakers are less than 8 Ohm each) you shouldn't run it that way since you can damage your receiver and/or speakers.
some info: http://www.termpro.com/articles/spkrz.html
(it's for car audio, but it's all the same)