There are a million (ok, more like a dozen) threads about this. Safari frequently reloading tabs, or crashing, particularly on the Air but also on other iOS7 devices.
Some speculate it is due to increased memory requirements of 64bit A7 on the Air, others dispute that because the same problem is being seen on 32bit devices like ipad4 and ipad3 running iOS7. Pick a side and join the rant.
Personally, I believe (hope) it is a memory MANAGEMENT issue or even just a Safari issue, as I don't see it happening in other memory-intensive apps... just in Safari itself, so I hope a 7.1 or 7.2 will bring a fix.
Time will tell. Search for the other threads and you'll find a lot of opinions and discussion.