MPU6050 + XBee/Serial = different readings

Hello Arduino Forum!

I looking for explanation of following situation:
Arduino UNO is connected to:

  • MPU6050 accelerometer/gyroscope (Vcc-3.3V,SDA-A4,SCL-A5,INT-2,Gnd-Gnd)
  • XBee S1 (5V-5V,Gnd-Gnd,TX-DIN)

I’m reading values with use of MPU6050.h and example sketches that were provided with this library.
Printing values with Serial.print() allows to send data through XBee and Serial Monitor.
Serial and XBees work at 9600 baud rate.

I upload example sketch connecting to Arduino with USB cable (also this is the power source), the data received through XBee is wrong - sensor at rest produces constantly growing values.
With the moment of opening Serial Monitor, sketch restarts of course, and readings in both Serial Monitor and XBee are correct and the same.

So question is: what changes in described setup when I open the Serial Monitor?
I will be happy to provide more information, if needed.