There is a write lock to the build queue and one event thread (:Events:pool-1-thread-8) grabs it to place a build in the queue. But it can't complete because it also needs a database connection. There is no database connection available and it waits to obtain it plus it keeps holding on to the queue write lock.
Because a lot of builds are scheduled for 8am many more threads need to obtain the write lock. Some of them hold a database connection already but need the write log. They would hold on to the db connections. Because users also browse bamboo even more db connections are needed to serve bamboo with the needed information.

There's bamboo's deadlock.