PH level

What does cause the PH level to spike from 7.4 to over 8.4 ??

give us a bit more info about your pool and post a current set of test results and we can answer that
But, assuming you have not added anything that would raise pH, usually it's new plaster and/or
high TA.
If you have a SWG then you can also add CYA too low or unit undersized for the pool.

Also, If your pH is in normal range, you shock the pool, and retest pH when at shock level you pH will have appeared to spike but really hasn't. High chlorine levels chemically interfere with the pH test and cause it to read high when it is not. Once the chlorine drops to normal range the pH the pH drops back down to where it really is.

The chlorine was at 2.0 that is not high. I have not added anything besides calcium Hypo chlorine in the last 4 weeks, since the pool got clear the water was balanced and everyday the PH was 7.4- 7.6. until today it shot up to over 8.4

How are you testing chlorine? If you are using DPD testing (comaprator with shades of red) or strips be aware that these can bleach out at high chlorine levels leading you to think the chlorine is much lower than it really is.
If this is the case it could explain the pH spike.

Also, what is your TA. High TA will cause the pH to rise. You did not state how often you are testing pH. If it is not daily (which it really should be) then your pH might not have 'spiked' but rather just climbed over time because of the TA.

Posting a full set of test results not done with strips would be very helpful, as would a discription of your pool and equipment, including any water features or other potential aerators of the water. Aeration of the water will cause pH to rise (and we use this fact to our advantage when we lower TA).