Though strikes are less common in France than they were a decade ago, they still happen. Normally they don't inconvenience too many people, and the French people generally support the rights of the workers so a little inconvenience is tolerated. However, the current train strike is causing a lot of pain to a lot of people, and most people don't really understand the reason for the strike.
We're not going to try to explain it either, sorry. But you should be aware that it is currently hard to know whether it will get better or worse over the next few days. Plan ahead, allow extra time for your transportation via SNCF (and to a lesser degree, TER), and watch the info site (English translation) for updates on the traffic forecast.