Suckerous habit makes me think these might have instead been a species of Rhus.

Clonal Oaks link.

HTH
Chris

Here is a link that might be useful: Oak Scrub

Although they even compare the "mottes" to aspen and buffalograss in the above article they do not state explicitly that the clumps of stems are resulting from suckerous growth like that of a sumac (or aspen), where a creeping underground rootstock grows outward indefinitely. Sprouting spread out over the normal root systems of old individual seedlings could account for the separate stems of the oaks.

Here is a link that might be useful: Wikipedia Mott Live Oak

Pin oaks grow readily here and you can see some sizeable specimens in older gardens and parks. 'Sizeable' is a key word - while there are dwarf varieties of pin oaks, they are not particularly common nor would they tend to make an ideal hedging plant.

I'd be incline to think the small trees/shrubs you saw were misidentified. Bboy offers a reasonable alternative and there could well be others. Any chance of a photo?

While I agree this could be a case of mistaken identity Suckering
Oaks are not all that rare or unusual. This is truly how several Quercus species proliferate and I disagree that they don't specifically state that the trees are from a common root system.

Regards
Chris

could you be thinking about an oak leaf hydrenga???

link below

ken

Here is a link that might be useful: link

"Common root system" does not automatically equate to creeping rootstock, in the manner of a sumac, aspen or bamboo, resulting in small specimens that "spread easily by underground roots".

When this was first brought up I wondered if someone might have been calling Gambel oak "pin oak". But the description of how the shrub in question was being used in gardens didn't seem at the time to fit. Maybe it actually did. Gambel oak is native to Utah and

spreads most rapidly from root sprouts that grow from vast underground structures called lignotubers. These reproductive characteristics often result in dense groves or thickets of the trees that often cover entire mountainsides

Here is a link that might be useful: Wikipedia Gambel Oak

"Common root system" does not automatically equate to creeping rootstock"

Clonal does imply suckering, not root grafts. Have you never
seen a Quercus viginiana or Q. fusiformis proliferating via
creeping rootstocks that sucker to form groves?

Regards
Chris

Clonal just means the whole unit is one plant genetically. Suckers (sprouts from the ground) can be coming up from more than one type of structure, including lignotubers. Anyway, there is a low growing oak native to Utah that spreads underground and might have been what was seen in a few yards. Next step is to find out if Gambel oak looks like what was seen and asked about.

thanks to all who answered, will continue searching. These were growing in a newer housing development and often planted on a raised irregular area purposely designed to show them off, usually out in the center or corner of a lawn.
pjj