The culprit is the emoji in the tweets. Some of the emojis have four bytes UTF-8 character.
The MYSQL's character set named UTF8 uses a maximum of three bytes per character and contains only BMP characters. As of MySQL 5.5.3, the UTF8MB4 character set uses a maximum of four bytes per character supports. http://dev.mysql.com/doc/refman/5.5/en/charset-unicode-utf8mb4.html