Why are there generally so many aborted transactions?

Hi!

A fresh Plone 5.0.5 installation (Plone Version does not matter) shows in debug mode a lot of aborted transactions. The ones listed here are from calling the welcome page, once:

2016-07-07 00:28:13 DEBUG txn.140148251096832 commit
2016-07-07 00:28:13 DEBUG txn.140148251096832 new transaction
2016-07-07 00:28:13 DEBUG txn.140148251096832 abort
2016-07-07 00:28:13 DEBUG txn.140148251096832 new transaction
2016-07-07 00:28:13 DEBUG txn.140147998586624 new transaction
2016-07-07 00:28:13 DEBUG txn.140148251096832 commit
2016-07-07 00:28:13 DEBUG txn.140148251096832 new transaction
2016-07-07 00:28:13 DEBUG txn.140148251096832 abort
2016-07-07 00:28:13 DEBUG txn.140148251096832 new transaction
2016-07-07 00:28:13 DEBUG txn.140147998586624 commit
2016-07-07 00:28:13 DEBUG txn.140147998586624 new transaction
2016-07-07 00:28:13 DEBUG txn.140147998586624 abort
2016-07-07 00:28:13 DEBUG txn.140148251096832 commit
2016-07-07 00:28:13 DEBUG txn.140148251096832 new transaction
2016-07-07 00:28:13 DEBUG txn.140148251096832 abort
2016-07-07 00:28:14 DEBUG txn.140148251096832 new transaction
2016-07-07 00:28:14 DEBUG txn.140147998586624 new transaction
2016-07-07 00:28:14 DEBUG txn.140148251096832 commit
2016-07-07 00:28:14 DEBUG txn.140148251096832 new transaction
2016-07-07 00:28:14 DEBUG txn.140148251096832 abort
2016-07-07 00:28:14 DEBUG txn.140148251096832 new transaction
2016-07-07 00:28:14 DEBUG txn.140147998586624 commit
2016-07-07 00:28:14 DEBUG txn.140147998586624 new transaction
2016-07-07 00:28:14 DEBUG txn.140147998586624 abort
2016-07-07 00:28:14 DEBUG txn.140147998586624 new transaction
2016-07-07 00:28:14 DEBUG txn.140148251096832 commit
2016-07-07 00:28:14 DEBUG txn.140148251096832 new transaction
2016-07-07 00:28:14 DEBUG txn.140148251096832 abort
2016-07-07 00:28:14 DEBUG txn.140147998586624 commit
2016-07-07 00:28:14 DEBUG txn.140147998586624 new transaction
2016-07-07 00:28:14 DEBUG txn.140147998586624 abort
2016-07-07 00:28:14 DEBUG txn.140147998586624 new transaction
2016-07-07 00:28:14 DEBUG txn.140147998586624 commit
2016-07-07 00:28:14 DEBUG txn.140147998586624 new transaction
2016-07-07 00:28:14 DEBUG txn.140147998586624 abort
2016-07-07 00:28:14 DEBUG txn.140147998586624 new transaction
2016-07-07 00:28:14 DEBUG txn.140147998586624 commit
2016-07-07 00:28:14 DEBUG txn.140147998586624 new transaction
2016-07-07 00:28:14 DEBUG txn.140147998586624 abort

I have seen this behavior on each installation I made in the last year on different systems, so it should be generic and not due to an installation error.

I am just curious, why are there so many transaction aborts? How can I track them? Is there a ZODB-DEBUG switch to monitor the transactions?
Surely Plone does some writing even on reads (page views) but why do so many of them fail?

Anyone an idea?

Best regards

Volker