A couple of years ago I read a very interesting and thought-provoking book:
Empires of the Silk Road by Christopher I. Beckwith (
http://www.indiana.edu/~ceus/faculty/beckwith.shtml). The subtitle of the book is "A History of Central Eurasia from the Bronze Age to the Present" and Beckwith paints - in broad strokes but with firm foundations in archaeology and linguistics - a fascinating picture of what happened all over Central Eurasia during the last five millennia.
The confines of what Central Eurasia encompasses have moved back and forth, and at times have encompassed everything between Portugal and Korea, and from Tibet and India to the Arctic.
Arabia is not included in this geographical definition, but when the Arabs boil out of their subcontinent they become completely part of the Eurasian mindset and behave in ways very similar to countless empire builders over the the past (more or less) known history.
Coincidentally Beckwith explores in the same chapter the expansion of the Arabs and of the Vikings, and the similarities are striking.
There were reasons for the Arabs to go and conquer, even if - at least at the beginnings - the idea of "empire may have not been there. These reasons include disruption to traditional trade routes, climatic changes, a power vacuum on the borders, long-lasting economic crisis and depopulation of the border regions. Religion is not there, or at least if is there is in the same way as the future king is sired by a god on a noble maiden, is abandoned to die and nourished by animals and when is grown up presents himself to the court of the old evil king, gains the loyalty of a group of heroes and go forth to depose the king, marry a princess and build an empire. In other words a founding myth, which worked - more or less word by word - for hundreds if not thousands of empire builders (including the legendary founder of Rome).
I find Beckwith convincing, and therefore I don't believe that Islam was a significant reason for Arab expansion. I would add that while pre-islamic Arabia was not unified it was not the nest of vipers Islamic hagiographers line to depict. Coastal Arabs and Bedouins from the interior were at times fighting but most of the time cooperated, were part of a society: Beckwith convincingly demolishes the myth of the predator nomad, stealing with violence the fruits of the labor of settled populations.
I do recommend this book without reservations. The only warning is that it is not an easy book to read belonging to academia more than to popular history. A review of the book can be found at
https://escholarship.org/uc/item/67z5m9d3#page-1