Why do miners seem to be using consecutive letters for the Coinbase transaction text? If you look at coin.dance, it looks like the coinbase transactions text start with consecutive letters. For example, block 478529 starts with "AM" and 478530 starts with "BM" and 478531 starts with "CM" and so on. This continues with DM, EM, ... all the way to ZM despite being mined by lots of different miners.

This looks coordinated in some way. Why is this? Can't the coinbase text be whatever the miner wants?
According to BIP 34 the coinbase transaction is required to start with the block height.  The format is a single byte giving the count of bytes to follow (currently 3) and then the block height in little-endian. See Why block height is required in coinbase? for more on the rationale.

478529 decimal is 0x074d41 in hex, so this block's coinbase must begin with the bytes 0x03 0x41 0x4d 0x07.  0x41 0x4d happen to be the ASCII codes for the letters A M.

Likewise, 478530 decimal is 0x074d42, so we start with 0x03 0x42 0x4d 0x07, and 0x42 0x4d is B M.