why ultrasonic module sends out 8 cycles ? and , why the trigger pulse is 10us?
The receiver and transmitter are mechanically tuned to the frequency, so it will take a few cycles for the amplitude to ring up to the maximum (the transmitter will ring up in amplitude as you drive it, and the receiver needs to 'hear' a number of cycles before it reaches full output, so you better drive it for enough cycles). It's also of no advantage to have too long a sequence of cycles.

That is why the designer programmed the microcontroller to output 8 cycles in particular, in answer to your first question.  



Read any reference on 2nd order systems for an explanation of Q and resonance. The center frequency of this mechanical resonance is typically specified to +/- 1kHz (+/-2.5%). Here is a typical one: 



Note the ringing specification of 1.2ms for this product, which implies a much higher Q.  It is a waterproof type and has too high a Q for good results in a ranging application. You can find more information in this answer. 



In answer to your second question, the 10us is probably to allow the firmware in the microcontroller to recognize the input. If they don't use an interrupt but rather a tight loop it might take that long to traverse the loop so a shorter pulse might be missed some of the time.