Is an extent allocation serialized or concurrent/parallel operation?

My case: multiple inserts to the same table (100K inserts in 6 secs). Everything went OK, but  almost 90 extents has been allocated, 64M each, almost 5GB together. Much, much  more than was in fact needed. At the and of the day it caused tablespace space allocation problems, so I need an explanation. My explanation is that extent allocation is executed in parallel, until first ~90 inserts requests has been fulfilled, then the rest goes. It seems reasonable, but I can't find any info in any Oracle docs about serialisation or parallelisation of this Oracle core functionality.
I'd be greateful for any, official or not, explanation. Or confirmation