The rigidity of anyone's pricing is a reflection of the costs associated with changes to the booking.
For example, if I work alone then I can be more fluid about how much additional time I'll support and what to charge (or not) for that. I could do that because I answer only for myself.
Yet, if I am booked full time with back to back events there is no flexibility because that time which you did not book in advance has been committed to another customer. I now speak for their interests in addition to my own. If you booked me until 5pm and I have another event starting at 7pm you're not going to get any overtime with a last minute request. You have to book that time in advance prior to me booking another sale or failing that, accept that another DJ would relieve me at the appointed hour and book in advance for that option.
This presents still another inflexible condition - that of employees. If I have a staff of people working for me - they expect to get paid for every hour they work, and they DO want to wok. They too, may also be booked on consecutive gigs and unavailable for overtime. They may also simply choose to decline on the basis of their own personal lives. If they do stay, I will need to make payroll taxes and insurance payments based on the hours they work. The total cost of an employee who earns $30/hr may be as high as $90/hr to the company. This is true no matter what the specific employee does for work. If a crew of techs was told the event strikes a 11pm and it continues for two more hours - they all still expect to be paid their full rate for the 2 additional hours of their time wasted while sitting at the loading dock.
Finally, is the issue of consecutive gigs. My employees want to earn as much money as possible. They make substantially more money doing back-to-back gigs on a Saturday than they could by doing overtime on just pone gig, or by doing a mid-day (crossover time) event like 3pm-8pm. For that reason, I would typically price overtime HIGHER than the normal rate, and either outright decline or charge a huge premium to do a crossover time.