Your rights notwithstanding, the government has the power to do such things under appropriate circumstances. First, you would have to be in violation of some ordinance, for instance in Columbus OH you are a violator if the grass is over 12". This should generate a notice informing you what the issue is and giving a deadline for remedy. If you don't comply by the deadline, they are then empowered to send out guys with tools, and the city will bill you for the work. You could call them and ask what the deal is. They might say "We put the notice on your gate", or "we mailed it to you". From a legal POV, the onus is on them to be sure that you're notified. It would be a good idea to verify that this isn't a scam.
[Addendum]
Bryan TX kindly provides a video about code enforcement, and gives a link where you can go directly to the section of interest (starting 0:43). Your description of the situation is at variance with what they say is the law (12"; 7 day advance notice whereafter they will correct the violation. They also say no notice is required for second violation within a year; $100 administration fee added to costs; lien will be placed on property if unpaid).
I assume that your back yard is publicly visible: they recognize that "when the area observed is plainly visible, from a vantage point where the Code Officer has the right to be there, there is no reasonable expectation of privacy". That could include visible from a neighbor's property if the inspector has permission from the neighbor to be there. Otherwise, there's a simmering 4th Amendment problem (assuming that they didn't get a search warrant).
