I thought about this for a while and looked around at other markets and it appears lots of eastbound to Europe is wide open right now, while westbound is booked solid. After some thinking, I think I might have an idea why. Americans take earlier vacations because of school ending in late May - so they head out in June sometime (which explains why June was booked solid eastbound). They spend a couple weeks then return in late June or July. Meanwhile Europeans start vacations in July, so you have both returning Americans and departing Europeans now filling up the westbound flights. In August the eastbound flights recover as the Europeans start to fly back home. I am curious to hear what others think of my theory.
Your theory is correct. This happens on many trans Atlantic flights, not just IND.
Americans go on vacation in May/June. Flights to Europe are full, and flights back not so much.
Americans come home from vacation in July/August. School is about to start. Flights back from Europe are full. Flights going to Europe not so much, because they are all coming home, to go back to school.
Europeans start their big vacation push in July/August. They fill up the already full inbound-from-Europe flights.
Europeans finish their vacation at the end of August. Flights to Europe will be full again. Flights back to the US not so much.
School is in full swing by September. Now it's time for the retirees, shoulder season bargain hunters, and business people to start flying. Bookings in both directions will pick up again, some days looking just as busy as in summer. This will last until November, when it drops off massively.
Then comes Christmas season, and the rush starts all over; flights will be packed both ways. This lasts until early January, when it really drops off, even more than in November. In the dead of winter, many airlines will cut their routes to 3X, 4X, 5X a week.
All February is quiet, with the exception of a few American holidays, and a few European school breaks. Then flights (especially going south...Florida) will be packed for a couple of days, before returning to normal winter low.
March comes, and people start flying again. The shoulder season crowd shows up, and the closer we get to Easter, the busier the flights get. Most airlines now will start to return to their 7X summer season schedule. The weeks before Easter, and leading up to Pentecost will be busy again, with the same pattern we see in July/August. Americans will fly over before Easter, coming back after Easter, and Europeans will fly over during their Pentecost break, going back afterwards. Depending when those holidays fall year after year, the rest of May will be quiet, until the start of Americans flying to Europe at the end of May. Rinse and repeat. Year after year.