|Although you wanted your assistant to call you if a problem arose, it sounds like your assistant was the problem that arose. You should meet with her as soon as you return to work and use some open-ended questions to learn more about her rationale for calling your manager. |
It is possible she thought the situation was so serious that it needed immediate managerial attention, and since you were not onsite, she may have believed that calling you would have lost vital time. Or perhaps she just did not want to bother you while you were away. However, it is also possible that she has been waiting for an opportunity to slam you in front of your manager, and that opportunity finally arrived. Once you have gathered your data, you can then determine an appropriate response.
As part of your questioning, be sure to find out the status of the problem that started this entire incident. If additional actions are needed, you should take them immediately. You should also meet with your manager to update him or her and put this entire matter into perspective and hopefully into history.