How? Simple, because they launched into those orbits.
Why? Well, first, let me explain what their orbits actually are.
IRAS (13777) and GGSE-4 (2828) are both in high-inclination orbits, 70° and 99°, respectively. The latter is slightly retrograde, as is common for sun-synchronous orbits. However, to fully understand in what plane they are orbiting, we need two elements, not just the inclination. (Why two? Because you need two measures to indicate a unique direction in 3 dimensions.)
This second element is their right ascension of the ascending node, or, more simply, at what longitude (not relative to the rotating surface but rather to to the First Point of Ares, a fixed point in the sky) they cross the Earth's equator traveling north. These are 214.5008° and 027.7275°, respectively.
You can see those are nearly 180° apart. Combined with the inclinations, which are 10° and 20° off of a true polar orbit but in different directions, and we've got two satellites in nearly the same orbit, except moving in opposite directions.
So back to why...
IRAS was launched into a sun-synchronous orbit because it is an infrared survey satellite and it is useful to always have the sun in the (nearly) same position when shooting infrared photos. That way, they're all taken at the same time of day & you don't have to consider the changes in lighting (as much) for remote sensing applications. GGSE was launched into a high subpolar orbit because it was a '60 spy sat and it needed to take photos of high-latitude Soviet things.
However, it seems IRAS was launched just at the right (wrong?) time of day that GGSE's orbit was directly overhead, but going the opposite direction that IRAS launched. Because their inclinations are similarly off of 90°, but in different directions, that gave their two orbits a very low relative inclination of around 10° (actually, technically, it's around 180, because they're going nearly opposite directions, but that's not my point here, my point is they're in the same plane).
So if IRAS had launched 12 hours later or earlier from when it did, they would have been going the same direction but with a 30° relative inclination difference instead of a 11° one.