Serialization errors in Postgres 9.4.0

Some time ago we upgraded from v. 8.4.4 to 9.4.0 and we are seeing errors in some regression tests.

LOCATION: exec_simple_query, postgres.c:887
ERROR: 40001: could not serialize access due to read/write dependencies among transactions

Detail can be one of these:
DETAIL: Reason code: Canceled on identification as a pivot, during commit attempt.
DETAIL: Reason code: Canceled on identification as a pivot, during write.
DETAIL: Reason code: Canceled on identification as a pivot, during conflict out checking.

Location is usually one of these:
LOCATION: PreCommit_CheckForSerializationFailure, predicate.c:4654
LOCATION: OnConflict_CheckForSerializationFailure, predicate.c:4600
LOCATION: CheckForSerializableConflictOut, predicate.c:3888

It may end up with
ERROR: 25P02: current transaction is aborted, commands ignored until end of transaction block

The database at this point is likely to have a barrage of read and write ops against few tables.
I understand that using serializable transaction we probably should be ready to retry,
but it would be helpful to understand why we did not see so many errors in the past.

Did something changed from 8.4.4 to 9.4.0?
Maybe Postgres has more aggressive predicate locking mechanism now?
Can it be that because of the small table size it performs sequential scan and locks the entire table?

Sorry if any of these questions are plain stupid.
Thanks,
Michael.

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