Why do both 太 (too) and 过于 (excessively) occur in 外国人有时候太过于开放 (foreigners are sometimes too excessively open)?
I'm going to convert my comments into an answer to offer another POV to the already given answer.

太过于 is a set phrase.
KEY defines:

太过于
too...

and KEY French defines it:

trop...

Perhaps this is similar to English usage of  too + overly. It's a phrase we often use in English too. A quick google search gives me results like: too overly complex, too overly dependent and too overly ambitious - seems to be about the same logic.
ICIBA also defines it as:

too much

giving examples:


She's too protective towards her children; she should let them be more independent.

她对孩子太过于爱护, 她应当给他们更多的独立性.

Cheese, like oil, makes too much of itself.

乳酪, 如同石油一样, 太过于引人注目了.

Don't be so quick to judge!

不要太过于武断!


Examples of usage (emphasis is my own):
Sina: 周航：很多时候的焦虑是太过于关注短期的东西

在搜狗CEO王小川、顺为资本投资合伙人周航和腾讯前CTO张志东圆桌论坛上，对于“科技发展与服务用户”的话题讨论上，周航认为，我们很多时候的焦虑，是太过于关注短期的东西了。

Sina: iOS 12加快淘汰速度，iPhone6确实太过于老旧了！！

而今天，还需要给大家再次强调，如今都已经是3月份了，如果按照苹果的更新速度，很快将会在秋季之前发布最新的iOS 12系统，这无疑会加快淘汰的速度，因为iPhone6太过于老旧了。

Google Books: 道解《黄帝内经》（上、下）

味太过于咸

and the kicker:
Google Books: 遇见成功的自己

对有些人来说，改变习惯的确是很难很难的事，因为他们太过于原谅自己，太过于迁就自己，太过于开脱自己，太过于娇纵自己。