I like to write calls against my stock to add a bit of revenue. I realize that I risk losing the stock but I got screwed this time. One of my stocks against which I had written calls was due to pay a dividend. I like the stock so I hold it in two different accounts at two different brokerage firms. On the very morning that the stock went ex-dividend, all of my calls IN BOTH ACCOUNTS were assigned! 
Now, it is my understanding that exercises are assigned randomly so it seems mighty strange that the options in both accounts held at different brokers were assigned and both on the morning that the stock went ex.
I can see why someone holding an in the money call would want to exercise if the stock was going to pay a nice dividend, but why on the morning that the stock went ex? Why not the day before? That person in theory would not know where the stock was coming from and in theory would not know if the stock being delivered to them was entitled to the dividend or not.
This smells of an insider deal - you know some play by those insidious floor traders or someone buried deep in the exchange infrastructure to steal my dividend.
Can anyone explain to me what is going on here (i.e. who made money on that deal and what was their setup)?

Now, it is my understanding that exercises are assigned randomly so it seems mighty strange that the options in both accounts held at different brokers were assigned and both on the morning that the stock went ex.
I can see why someone holding an in the money call would want to exercise if the stock was going to pay a nice dividend, but why on the morning that the stock went ex? Why not the day before? That person in theory would not know where the stock was coming from and in theory would not know if the stock being delivered to them was entitled to the dividend or not.
This smells of an insider deal - you know some play by those insidious floor traders or someone buried deep in the exchange infrastructure to steal my dividend.
Can anyone explain to me what is going on here (i.e. who made money on that deal and what was their setup)?