Google detected you were not running Google Chrome (with which they can easily profile you) so you end up being punished for that. Which leads to Google Chrome being the preferred product. They made up some punishment system and reason to cover that up.

Just kiddin'. Sometimes, I end up having to do unreasonably more captchas. The reason is that the system verified I am human, and it wants me to cooperate with it for free in order to authenticate more captchas. Machine learning, they call it. I call it un(der)paid labour. Never had 15 in a row though, so I suppose the already mentioned reason is more plausible.