TS is not alone, I, too have 2 of my lightstands fall during a shoot.
Luckily my flash did not spoil as both of them are on receivers thus the receivers took the impact and broke into 2.
At first, thought that my lightstand are not steady and stable enough. But after some consultation at sgcamerastore, the shop keeper explained to me that even with manfrotto lightstand, it will still topple when a gust of strong wind comes. The only way to prevent it from topple, is to get those sand bag as a weight, or keep 2 X 2L of water on 1 lightstand.
For me, I ended up using the Sunkist orange juice 2L bottle, 2 of them for each of my lightstand. And now, my lightstand stopped to topple during my shoots.