Main thing is lag, the servers give you some leway with regard to jumping because too rigid a timer would hit too many false positives on laggy connections. The speed hacks push the tolerance to the limit, so the chance of getting d/c are greater. It's like, if you have sent in so many jump packets travelling so far in a set time period, there is an upper limit to what the server will allow. Putting it another way, imagine you have the crappyest of crappy connections that can lag out at odd times, you could jump 3 times during a lag spike and so the server recieves all 3 jump packets at once when the spike clears, that would look to the server like a speed hack. There is a timer running on the client and the server that tries to remain synced in order to reduce this problem, but too many jump packets in a certain period of time and BANG! you're d/c.