Ambassador Sondland is ambassador to the EU. So why was he involved in Ukraine, a country that is not even in the EU? I understand that Ukraine is considering joining the EU but why was he so heavily involved there? Is that standard practice or highly irregular?
Yes, it's "irregular" (but "not as outlandish as it could be")
Ambassador Taylor, in the opening statement of his public testimony before congress referred to the diplomatic channel of which Sondland was a part as the irregular channel (quoting from the transcription by rev.com):
At the same time, however, I encountered an irregular, informal channel of US policy-making with respect to Ukraine, unaccountable to Congress, a channel that included then-Special Envoy Kurt Volker, US Ambassador to the European Union Gordon Sondland, Secretary of Energy Rick Perry, White House Chief of Staff Mick Mulvaney, and, as I subsequently learned, Mr. Giuliani. I was clearly in the regular channel, but I was also in the irregular one to the extent that Ambassadors Volker and Sondland including me in certain conversations. Although this irregular channel was well-connected in Washington, it operated mostly outside of official State Department channels.
The counsel for the minority asked Ambassador Taylor about the irregular channel (see this clip by PBS):
I want to turn to the discussion of the irregular channel you describe. And, in fairness, this irregular channel of diplomacy, it's not as outlandish as, as it could be, is that correct?
And Ambassador Taylor replies, smiling:
It is not as outlandish as it could be
And after a bit the counsel for the minority asks:
And the second member of the irregular channel is Ambassador Sondland, who is senate confirmed, ambassador to the EU, so his involvement here, while not necessarily part of his official duties to the EU, it certainly is not outlandish for him to be interested and engaged pursuant to the president or Secretary Pompeo's direction.
And Taylor replies:
It's a little unusual for the US ambassador to the EU to play a role in Ukraine policy.
The counsel for the minority then goes on to ask:
And you know it might be irregular but it's certainly not outlandish.
Bill Taylor nods in reply.
Ambassador Sondland is a hotel magnate who Donald Trump elevated to Ambassador to the EU as a reward for large campaign contributions during Trump's 2016 presidential campaign. The actual Ambassador to Ukraine, Maria Yovanovitch, was a career diplomat and a longtime member of the US Foreign Service with a degree in Russian Studies from Princeton.
While Ambassador Sondland was not involved with anything but the most rudimentary business involving Ukraine, he is currently indebted to Trump via the contribution-for-position quid pro quo they engaged in, and thus served as a much more trustworthy channel for Trump.
The question itself asks the answerers to project interpretations onto a situation that none of us have been a part of and all of us have skewed interpretations for relative to the sources of information we derive our views from.
Is it uncommon that an Ambassador of another region was involved in another Ambassador's matters at the behest of the US President? Yes. Is it illegal? No. Was there potential reason to do so? Yes, given the behavior of Maria Yovanovitch.
Understand that US Ambassadors work for the Executive branch of government (the President). An Ambassador working counter to the agenda of the head of the Executive Branch, and very publicly at that, might be principled but certainly isn't appropriate. Just as Gen. Petraeus was removed by Obama for publicly speaking out against the President, it is entirely legitimate for Trump to do the same.
Working with the US government for 20 years and in large corporations, it is not at all uncommon for someone who can be trusted to be put in an 'oversight' or 'dual-hatted' position to address a problematic institution--in this case, the Ukrainian team.