These segments in the timeline are associated with blocking times that are categorized as Synchronization. When a thread is marked as blocked on synchronization, one of these things is implied:
The execution of the thread may have resulted in a call to a well-known thread synchronization API such as EnterCriticalSection() or WaitForSingleObject().
The API matching algorithm cannot be totally comprehensive, and therefore some APIs that could be mapped to other categories may also appear as synchronization because a frame in the call stack eventually reached an underlying kernel blocking primitive that was mapped to this category.
To understand the underlying cause for a thread blocking event, carefully examine the blocking call stacks and profile reports.