That started happening with my dfs around the 4 or 5 month mark. It was almost 100% of the time for a while there. Things seemed to have slowed now. He is 6 1/2 months and wakes up screaming/crying only about 50-60% of the time. He does usually calm down a minute or two after we pick him up, especially if we smile really big and start talking with him in a cheerful voice while distracting him with something fun, but on occassion he keeps on going for several minutes.
Around the same time it started happening, someone else with a baby about the same age posted this same question. Lots of folks had suggestions-- gas, reflux (he does have reflux), teething (he did start not long before he began teething for the first time...but then again, it has gotten better over time even though recently he was cutting 4 teeth at one time!), bad dreams, etc. Unfortunately, we never were able to nail down exactly what was happening in a consistent manner with the crying and what would fix it.
I'm glad it is slowing down. Since it seems common around this age, I wonder if it is teething related or maybe just related to the frustration of this developmental stage. They are learning so much, and at the same time, still so limited in terms of mobility. The cries sound more like pain cries than frustration, but I just wonder...
I'm pro-adoption reform, but not anti-adoption.