I haven't been introduced to RTP(Real-time Transfer Protocol), so I searched wikipedia for the answer and this is what I found.
The Transmission Control Protocol (TCP), although standardized for RTP use, is not normally used in RTP application because TCP favors reliability over timeliness. Instead the majority of the RTP implementations are built on the User Datagram Protocol (UDP)
According to this, I guess what's troubling you is why it uses UDP, and not TCP. Well, I just wrote you the answer - it is implemented so to favor timeliness over reliability. For further reading you may follow my way and ask wikipedia. I presume you'll find the chapter "OVERVIEW" most informative one.
If you're asking why are your packets experiencing problems, and having bad checksums, it could be for various reasons. What I'm going to say now relies on the fact that you said it happens SOMETIMES
Those corrupted packets could be product of someone's bad intentions, but as well, could be just regular packets that were exposed to interference of any kind(trams/electrical trains passing by your home etc.). After all, that's what the checksums are for - to check if packets were corrupted during transport. At the and I would like to add, I think that, what you are experiencing is normal and I doubt it is anyone's malicious intention.
If it's happening all too often, and significantly
reducing your throughput, you should contact your ISP to help you reduce the interference.