This is because the default behaviour for parental relationships is to cascade the action on a parent to all its related records. If we look at the number of parental relationships for the account entity you start to get an indication of how much cascading can happen and the potential performance impact. In particular the impact of a Parent Account and the number of related contacts.
Something else to be aware of is that when the ownership is cascaded to activities then the modified date is updated to the current date and this can be confusing for the users when they view the social pane. There is no way to prevent this and as change of ownership is considered a modification.
You can configure the cascading behaviour of a relationship and the options are as follows