[blaze8902]

Well, 2x4s are anisotropic, so orientation of the grain is important, as is the direction of the force applied. I'd say either your wood had an un-surreptitiously placed grain, or your team members have got strong arms.
How long was your 2x4, and where was it supported? a 4' 2x4, suspended on both sides will break at about 317lbs, assuming you're breaking it by applying the force on the 4" side, where it will be easier. If you threw it at the 2" side, it's more like 740 lbs. These calculations were done based on the assumption that the grain of the wood ran lengthwise down the length, as this is the most common. If your grain was another way, then it'd be easier to break. If that's the case, you should consider finding a new wood supplier, because that's both unusual and bad for construction.

Edit: The length, position, grain orientation, and support location are all factors. If you would like, I can calculate the force required to break if you supply me with this information. (I'm no expert on wood, so I can't say for sure just how accurate I can be if you say that your grain was not oriented lengthwise.)