I've seen a lot of my Chelsea supporting friends/colleagues be pissed at this decision.
However, if you look at the results since mid of last season, Chelsea have hardly looked like a team looking to achieve great things. I believe his record in the last 25 games was worse than the record Lampard had in his last 25 when he was sacked. Most of your underlyings, I believe, have been trending downwards. Add to that, your defenders are basically your goalscorers, or in other words, the tactics going forward don't seem to be sustainable.
Of course, the transfer window seemed like a mess where it looked like Boehly was acting like a 5 year old kid in candy shop, but if you remove emotion out of all of it, it's not the most unreasonable decision, certainly not something I'd not expect from Chelsea