IMO, this acquisition could have been motivated by 1 or more of the following: (1) Need them in house. Many larger companies have been contracting their app development to ISVs. Some ISVs have been quietly purchased by their largest customers, as mobile has become more "core". Crashlytics was a toolmaker that Twitter and some others used. Perhaps, Twitter thought these tools were "core", or too important to not have…
Looks to me like a talent acquisition. While Crashlytics was likely popular, I think they likely had limited revenue opportunities given Hockey App and Crittercism.
Really - it's a race to the bottom on these crash services. Switching costs would seem to be pretty low - so my guess is that the VCs saw a quick out.
Congrats to the team for the acquisition.