A good exemple of ranking problem.

[Kamikaze28]

The effect you are describing is not surprising to me. Let me give a little context first.

The system is supposed to gauge your skill and the only source of information it gets are your wins and losses and the skill rating of your opponents at the time the respective game was finished.
Other than the system's representation of your skill (the hidden matchmaking rating), we can imagine you have your true skill. That is to say the sum of your cognitive faculties, experience and strategic thinking which, unfortunately, can hardly be represented as a value or set of values.
Nevertheless, the system tries its best to keep the discrepancy between your true skill and the matchmaking rating as small as possible. This is tricky because it is easy to over- or underestimate you based on a winning or losing streak. Some 'stickiness' helps to smoothen your matches. But too much 'stickiness' will result in frustration because no matter how much you improve and how many faces you melt in your league, the system will not be convinced that a promotion is in order.

What you did with the creation of a second account is to re-initialize the skill gauging process. And because your true skill has improved since you started your first account, the system will happily place you in a higher league, because it doesn't know that you once had a lower skill rating.

A little fine tuning may be required to find the right amount of 'stickiness'.