This can happen for several reasons, all fixable.
1) The most common is fertilizing a dry plant with too strong a strength fertilizer solution. The fertilizer goes to the tips and they die back.
2) Root die back to the point that they cannot carry water/nutrients to the leaves.
3) Lack of water.
You did not say what condition the roots were in when you repotted. Were they firm? If not, you may have filled your new pot with rotten roots which will cause you huge problems. Also if you used Miracle Grow Soil and not a bark mix, this will be too heavy for Cymbidiums and will also cause problems.
If you are uncertain how to repot or divide a Cym. I will be glad to send you a culture sheet used by the Gold Coast Cymbidium Growers, a Branch of the Cymbidium Society of America (CSA). Just send a PM to
goldcoastcymgrowers@gmail.com with an addy they can send the printout to. Many growers on this Forum have found it very useful.
Buying plants from the box stores is always an iffy situation as the staff haven't a clue how to handle them and generally over water them. If you can get them when they first come in, you are usually OK. However, they need to be repotted as soon as they are out of bloom in the case of Cyms because the mix they are in may not be the best for your situation.