" The girt bar on the 737 is a crappy little rod that has to be engaged in two slots by hand and disengaged to disarm the door.
A distinct possibility is that the girt bar was not properly in the slots
I don't know about the NG, but, on the 2/3/400 series the girt bar was hardly a "crappy little rod". fairly light in construction yes, but certainly not as you describe it.
The girt bar attachment points on the floor however, were prone to sticking due to a combination of usage and the general "rubbish" that accumulates on entry door / galley floors, no matter how extensively the area is cleaned.
Having deployed plenty of "time ex." slides over the years, we can only be thankful, that, whatever the causal factor (s) there were no injuries or fatalities given the rapidity of a slide deployment which is the intention after all when being used for the purpose they were designed for.
It will be interesting to learn what did cause the activation therefore.