AVR Timer1 OCR1A controls TIMER1_COMPB_vect interrupt!
By setting TCCR1B |= (1 &lt;&lt; WGM12); you operate the timer in CTC mode counting from 0 up to the value assigned to OCR1A and back to zero again.

With the given value of OCR1A = 10000; the timer counts 0-10000, 0-10000...
For each 0-10000 count you get a Compare B Match Interrupt when the counter reaches the value of 100 (because you have set OCR1B = 100).

By changing OCR1A you essentially change the upper count limit of the timer which means that you also change the duration of each count cycle, so the Compare B Match Interrupt occurs faster.

As an example, if you set OCR1A = 5000; the Compare B Match Interrupt occurs twice as fast compared to when OCR1A = 10000; because the timer restarts the count when it reaches 5000 instead of 10000 so the counter period shortens.

As a side-note, make sure that you include in your code the interrupt handler functions  for all the enabled interrupts (ISR(TIMER1_COMPA_vect) seems to be missing) or resets may occur.