Is it normal for a sprinkler system to increase water pressure to a house?
It can, if you live in a hot sunny climate and there is no one-way valve to the sprinkler system. Cold water in, or on their way to the sprinkler valves will be heated by the ground or sun, and expand will a little causing significant back pressure.

If you have ever left your garden hose filled with water with the hose bib shut off on a sunny day, then pulled the trigger on the hose end, you will see the same effect. Lots of water pressure.