Denver has five to six arboreal inspecotrs that drive around looking for hazards and addressing complaints. The city is divided into five to six areas and each inspector is assigned their own area to inspect. I have been getting a lot of estimate requests regarding ROW Silver Maples due to city notices. The Denver Post recently had an article stating that lots of Silver maples are reaching maturity 70-80 years old and are hollow, therfore, dangerous.
The criteria for an inspector to deem a tree hazardous due to being hollow is, for every 10 inches of trunk diameter, there has to be at least one inch of sound wood. they determine this by using an increment borer, or at lest they are supposed to. Another less precise method is to "sound" a tree, meaning hitting the trunk with a rubber mallet and listening to the sound it makes. You should ask the inspector in your area how he/she tested the tree.
To work on any ROW tree in Denver, the contractor must pull a permit from the city, this includes trimming, removing and planting. I would also ask the contractor who trimmed the trees to show you the permit for trimming. The permit is issued by the inspector in your area. If the trees are so hollow, a permit to trim should not have been issued, or the contractor should have noticed while trimming and notified you, unless there were no visible signs of decay.
I have contested the city several times regarding the viability of trees in question, won a few, and lost a few, a lot depends on the inspector. I talked with one inspector last week who stated that there is a new law being proposed that all trees in the front yard in denver will be under the jurisdiction of the Denver forestry department, meaning you cant do anything in your own front yard without a permit.
At the minimum, the contractor who trimmed the trees should offer you the money spent trimming as credit toward removal.