I don't know about the most common, but in my experience it's certainly accounts for a significant number of failures. We also see a smaller number of LEDs which appear to have badly overheated, enough to have discoloured and distorted the plastic lens.
My experience is with LED stage lighting fixtures, so we're working with single-colour, white or 3-colour LEDs - the same seems to be true of all of them. The LEDs are typically soldered to an aluminium-substrate PCB - presumably this is the same construction as with your streetlamps? I've always assumed that the failure was brought on by thermal cycling and dissimilar expansion between the board and the LED. In which case, perhaps some lead forms are more susceptible to this than others?
Certainly I'd think that the reliability that we see from stage fixtures would be inadiquate for street lighting applications. I imagine (hope?) that streetlights must be run with a wider safety margin...