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There can be multiple reasons for errors in updating data conditionally in MongoDB, some of which are:

1. Incorrect syntax: The update query needs to be written in the correct syntax, failing which the database won't be able to understand the query and a syntax error may be thrown.

2. Incorrect data type: The data type of the update query needs to match the data type of the field being updated. If the data types don't match, the update may not occur and an error may be thrown.

3. Data validation rules: If the update violates any data validation rules, such as unique indexes, the update may fail and an error may be thrown.

4. Insufficient privileges: The user updating the data may not have sufficient privileges to update the data. In such cases, an error indicating "not authorized" may be thrown.

5. Inconsistent data: In some cases, the data being updated may be inconsistent, i.e., some documents may have one format and the others a different one. In such scenarios, the update query may fail, and an error indicating "cannot extract field" may appear.