As per chic's post, the reasons officially are:
Conditions, pay & pensions.
Let's take conditions first of all. Really? I've heard this for over a decade now. Why strike now? I think most of the public believe this is simply a smokescreen for the other two reasons. I like many suspect if the government moved on the other two reasons that teachers would happily stop strike action and return to work.
So what about pay? Teacher's dislike the idea of being rewarded for good performance and they fear that their boss (ie. the head) will set their targets unfairly.
Sorry but that's pretty much how the rest of industry works. I get targets to meet. My boss gets his targets from his boss. There are elements of my objectives I hate and I also have grave concerns about the way our team are measured. Frankly it seems like another issue that the rest of society already put up with and teacher's are resisting. There probably will be some teachers with unrealistic targets, most will probably be fair. Some will be too easy. Such is life. It doesn't make the entire principle unfair. In short parents want high performing teachers and the way it comes across is that teachers don't want to be held accountable for their performance.
So lastly pension. Again this is something that the rest of society have already had to stomach. I do have a certain amount of sympathy that the pension you signed up for in good faith is changing for the worse. But ultimately the government is skint and cannot afford the status quo. To put it another way. Let's say it was a private company. It promised pension x to it's employees during the good times when money was plentiful. Now it's making a loss. It can't pay that pension anymore. What's the point in going on strike? What will it change? It simply can't pay money it doesn't have!
Sadly I think it's mainly the latter as to why teacher's are really striking. Go for it. Get it out of your system. Vent your spleen, show your anger then get back to work please. Our kids deserve better than to be saddled with crippling debts in the future to pay unaffordable pensions promised now.