In addition to the other answers, phones using a keyboard with 'next word' suggestion shortcuts will learn from your entry into text fields, but typically not password fields. If you have a password of common English words, it is very plausible that a phone will offer up the next word in your password if you type it, and have previously typed your password into a text field.
Only a serious issue if your phone is compromised or the database of your word predictions are compromised or shared insecurely (such as in a heuristic learning program that shares your phrases with the keyboard vendor for better learning)