There are a few infrequent events where people exercise early:
1) They think that they are receiving income by receiving the dividend so they exercise
2) They want the stock
3) They throw away time premium by exercising
4) Risk Arbitrage, hoping the stock recovers to the pre adjusted ex-div close
A lot of web sites as well as people write that if the time premium of an ITM call is less than the dividend then the call will be exercised early. That's not true because there's no arb available to lock the difference. With time premium remaining, there is no incentive for the call owner to execise the call early to acquire your stock because by doing so, he is throwing away any remaining time premium. It makes more sense for him to sell the call for salvage value and if he actually wants the stock, buy it outright.
Where the dividend comes into play is if the time premium of an ITM put is less than the amount of the dividend. Then the arb is to buy the put and buy the stock and exercise the put on the ex-dividend date and then receive the dividend on the Pay Date.
Because owners of ITM calls do not receive the dividend, they tend to sell before the ex-div date when share price will be reduced by the amount of the dividend. If that selling drives the premium below parity, it presents an opportunity for Discount Arbitrage because the call buyer can exercise the call while shorting the stock, thereby locking in the discount to parity. It may sound like splitting hairs but the dividend doesn't cause the exercise. The arb arises if selling of ITM calls prior to a pending dividend drives the premium below parity - then the arb situation arises.
If there is no dividend then any situation where the option trades below parity presents the arb opportunity.
For example, suppose I own the XYZ Nov $35 call and with the stock at $40, the call is trading at $4.75 x $5.20. The intrinsic value of the call is 5 points. I could place an order to sell at a better price but no one is likely to give me $5. If no takers at a better price, to avoid taking the 25 cent haircut, I exercise the call to buy XYZ at $35 and simultaneoulsy sell 100 shares per at $40, grossing $5. Why give the MM the opportunity to do this rather than doing it yourself? And now, someone gets assigned and has to sell their stock (or go short). That could be you where the short call of your condor (or other strategy) is trading below parity.
One area I'm not sure about is the assignment process of the OCC wheel. I once had some naked OTM MSO calls with a dollar of time premium remaining assigned early. Free money. I covered the stock and immediately re-sold the short calls. I have no clue if the "wheel" process has any specific parameters regarding time premium remaining. It just might have been an error on their part. If not an error then randomness might be another reason for early assignment. Anyone have any feedback regarding this?