Why Did my Invisalign Crack?

While the aligners are plenty strong for their purpose, they are not designed for indefinite use, like a retainer. Given enough time, all aligners would crack or break.
If you are a "high mileage" patient, like one that grinds, clenches, has many buttons (thus greater retention and challenge to remove) you will see cracking much sooner. If the cracks are happening at day 11 out of 14, I wouldn't worry too much about it. If they are happening on day 2 out of 14, then another strategy should be considered.

Sometimes we see cracks develop in Invisalign aligners due to the stress of inserting & removing the aligners frequently.   One tip is to NOT always put the aligner in & out in the same manner.  For example, don't always grab the same area & pull on that area first when removing the aligner.  Switch it up.  The plastic does fatigue & wear out.  But hopefully, this will help you get through the 2 weeks necessary for each aligner.  

The attachments you have glued on to your teeth may cause the aligners to be more difficult to remove in specific areas. If an excessive amount of flexing or torquing is required to remove your aligners on a regular basis then you may begin to see the effects manifested in cracks and tearing. This is caused by having 2 or more attachments clustered together in one area. Cracking can also be caused by chewing on aligners. Clenching teeth at night can be a possibility too even though you may not realize it.