1195725856 is GET[space] encoded as a big-endian, four-byte integer (see here for more information on how that works). This indicates that HTTP traffic is being sent to Kafka port 9092, but Kafka doesn’t accept HTTP traffic, it only accepts its own protocol (which takes the first four bytes as the receive size, hence the error).
Since the error is received on startup, it is likely benign and may indicate a scanning service or similar on your network scanning ports with protocols that Kafka doesn’t understand.
In order to find the cause, you can find where the HTTP traffic is coming from using tcpdump:
tcpdump -i any -w trap.pcap dst port 9092
# ...wait for logs to appear again, then ^C...
tcpdump -qX -r trap.pcap | less +/HEAD
Overall though, this is probably annoying but harmless. At least Kafka isn’t actually allocating/dirtying the memory. 🙂