So you are playing Dota 2 ver 7+ ?

Have you checked the graphical requirements of the game?

Have you checked your connection to the server? ( not just your internet connection, because the server is sometimes slow )

Have you patched your game correctly?

The casting animation, attack animation, skill animation namely any animation will cause a spike in your kbps. If more than one animation is present and can be seen on screen this will spike your games kbps. If your connection speed to the server is not that good it will spike.

I have had this happen to me a lot of times. This also used to happen a lot in Dota 1. Spectre, Zeus and most massive moving animation skills will stutter and spike your game.

Also since Dota 2 loads different characters with lots of different cosmetic items. Which also animate differently from each other so you also get a spike there.

So sometimes its your connection which is trying to catch up to the animation from the server. Other times its just the game and a patch was not installed correctly and has caused some kind of problem. Most times its just the computer not being able to handle the graphics.

Edit: You should also check if you have any background applications working during gameplay. Check if the Steam client is downloading something while in the background. Also try to test your ping to the server using this: https://steamcommunity.com/sharedfiles/filedetails/?id=217075877