What is causing these ceiling cracks?
There is a very good chance that these cracks were covered up before you bought the house.   From the general looks of it, they look cosmetic.   

What is causing them is the change in temperature of the attic.

Now the co-conspirators here could also be a little moisture (I am doubting the venting of bathrooms are the culprits on that front), could be bad tape job, and I am going to guess the biggest is that extra weight.   

I am going to guess you have a standard attic with joists either 18 or 24" spaced.  I am going to guess that they put a pad down for the HVAC but the rest of the attic has no subfloor.   It is this subfloor that helps bind and disperse weight through the joist system causing less variance during seasonal movements.   Me thinks these cracks started appearing 7 years ago and they basically ignored them before.   Looking at your pictures and I would need better ones but looks like there are perpendicular mud or paint strokes by the cracks (should have been taped).

So what can you do?


You can put in cross braces for the joists to disperse movement.   You could certainly put in a subfloor but that might be overkill.   
Then you also want to make sure this space is very very insulated.   This is probably the easiest thing to do.   
Then when the cracks are at the worst, and you have insulated and put in cross braces around the joists in question - tape and mud.   And in a different question you can get techniques and mud types best for this.   


Or you can just look at your cracks because it is just drywall.   You have to understand this isn't a huge deal, you need more information to fix it right and quite frankly to fix it right might cost a bit and I doubt someone will give you a 100% warranty the cracks won't somewhat reappear.