Drush Segmentation fault when upgrading from Drupal 6 to Drupal 7
Drupal is very sensitive to the order you do things when doing a major system upgrade.  You must follow the instructions in UPGRADE.txt exactly; in particular, if you run updatedb just once, after upgrading your core and all of your contrib code, then the upgrade is sure to go wrong.  The symptoms can vary; I would expect that this is probably what is causing your segmentation fault.

If you look at the UPGRADE.txt instructions closely, you will see that you need to run updatedb on core only, with no contrib modules enabled; after that, you must upgrade your contrib modules one at a time, upgrading their code and then running updatedb for each one.  If a module that has a schema in the database has update hooks that need to run, then updatedb will run them, even if the module is not enabled, so you must be extremely exacting in your code management, or the upgrade will fail.  It is also critical that you upgrade your modules in the right order, because a module will clearly need to have its dependencies available in order to successfully complete its updatedb processes.  If you have a lot of modules, this process can be extremely tedious.

The Drush Site Upgrade command can be used to automate this process to a large degree.  There are still some things that you need to do manually, but it will walk you through the UPGRADE.txt instructions one step at a time, offering to do most of the steps for you automatically.  It will even give you advice about replacement modules for some contrib projects not available in D7, and will run the content migrate code to insure that you CCK content gets upgraded too.

If you go through all of the steps faithfully, and you are still getting a segmentation fault, it could be that your system is running out of memory.  Adjust your settings in php.ini to adjust.  If this still doesn't work, you could be encountering a bug in a contrib module.  When this happens, you can try asking for support in the issue queue of the particular module that is causing you problems.  If this does not get you anywhere, your final resort is to build a new Drupal 7 site from scratch using similar modules, and migrate your data using Drupal to Drupal migration.  In the futue, Drush Site Upgrade will also support the d2d upgrade path, but for now you'll need to do it manually.

Postscript: Understanding what a segmentation fault is is not really important to solving your problem, but there is a good definition on Wikipedia here: http://en.wikipedia.org/wiki/Segmentation_fault.