tl;dr it's because of water. (It's always water isn't it?). This is the North Crimean Canal, dammed up since the Russian invasion of Crimea. Crimea is rapidly running out of water, and Russia needs this canal flowing again.
This is the canal's route, running from the Dnieper river across Crimea.
In 2014, after the Russian invasion, Ukraine decided the best option would be to cut Crimea off, so they dammed the canal.
Crimea is now running dangerously short on water, and the situation is quickly becoming untenable for the Russian occupation. ukrinform.net/rubric-polytic…
The dam is currently 16km from the border
BUT, the mouth of the canal is over 60km from the border
In order to successfully restore water to Crimea, Russia would probably have to retake this area
So, imagine if your this cheery looking fellow, Gen. Oleg Salûkov. Putin just popped into your office and told you that the canal must be taken ASAP. What would you do?
I'm going to work off the great @Liveuamap now. You already have Ukrainian defensive positions set up against rebel forces in the west, but they aren't in the best condition. In the south, Ukrainian troops could move in quickly at any sign of military mobilization. So no surprise
@Liveuamap If you know you can't surprise them, you have to overwhelm them. First, you have to tie down ready reserves in a quagmire to the east with the rebels. Give the rebels a bunch of armored vehicles (which has been happening over the past month) and make them distract the Ukrainians.
@Liveuamap Now, you probably have about enough troops to meet the 3:1 ratio against a dug in opponent, lets look at the local area.
@Liveuamap Good, this looks like a ready-made trap for any defender. Knock out the bridges over the Dneiper and the only way in or out is to the east. Make a hard push to the river and you can probably cut off most of the Ukrainian defenders. Then all you have to do is mop up any resistance
@Liveuamap From this, you can see how the Russian troop movements fit in. Sprinkle some airborne troops to capture important objectives, and the plan starts to come together. All in all, a very attractive proposition was probably placed on Putin's desk.
@Liveuamap Quick, easy war, complete destruction of your enemies' front line troops, and restoration of the water supply to Crimea.
Earlier today, Russian forces reportedly started a new offensive in the Kharkiv area.
Russian units pushed through the border settlement of Pyl'na, but took heavy Ukrainian fire just north of the town, losing at least 3 BMPs and an MT-LB in Russian territory.
In the Black Sea, Russian forces spotted a Ukrainian USV armed with a twin rail launcher for R-73 all-aspect IR homing air-to-air missiles.
Based on the footage, some Ukrainian USVs now possess a nascent SAM capability, likely able to target low and slow Russian helicopters.
Russian Milblogger Fighterbomber addressed the situation earlier today.
"the Ukrainians were able to somehow connect the rocket with the USV telecontrol systems and are trying to work with them... At the moment, the most effective means against USVs are helicopters.
Russian forces were able to destroy this one off the coast of Crimea: